Functional Multilayer Organic Thin Films Fabricated by Layerby-Layer Sequential Adsorption Technique
by
Dongsik Yoo
Bachelor of Science in Engineering
Seoul National University (1991)
Submitted to the Department of Materials Science and Engineering and
the Program in Polymer Science and Technology
in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1997
© Massachusetts Institute of Technology 1997
All Rights Reserved
Signature of Author
Department of Materials Science and Engineering
May 2, 1997
7-)
Certified by
Michael F. Rubner
TD! Professor of Polymer Science
I
Thesis Supervisor
Accepted by
Linn W. Ho bs
John F. Elliott Professor of Materials
Chair, Departmental Committee on Graduate Students
JUN 1 6 1997
Lr'RARI'ES
Science
Functional Multilayer Organic Thin Films Fabricated by Layerby-Layer Sequential Adsorption Technique
Dongsik Yoo
Submitted to the Department of Materials Science and Engineering and
the Program in Polymer Science and Technology on May 2, 1997
in Partial Fulfillment of the Requirements for the Degree of Doctor of
Philosophy in Polymer Science.
ABSTRACT
A novel layer-by-layer sequential adsorption technique based on electrostatic
interaction is a powerful way to fabricate multilayer organic thin films. The
advantages of this technique include control over the number of layers, the
architecture of the multilayers, and their ease of fabrication. By controlling
the conditions of the polyion dipping solutions, such as salt concentration,
solution pH, and polymer concentration, the thickness of an individual layer
in the multilayer can be systematically manipulated. Solution pH and salt
content affect the degree of ionization and the repulsive forces between the
same charged segments of the polyion, respectively. The technique has been
used to modify surface properties of various substrates on a molecular level.
Contact angle measurements show that the surface wettability can be
manipulated in a layer-by-layer manner to produce very stable surfaces with
characteristics ranging from hydrophobic to hydrophilic. The sequential
adsorption of a number of different functional dye molecules has been
accomplished via the alternate spontaneous adsorption of polyelectrolytes
and ionic dyes from dilute solutions. Multilayer thin films containing such
functional dyes as pH indicator dyes, infrared absorbing dyes, porphyrin dyes,
and various fluorescent dyes have been successfully fabricated and their
electrical and optical properties examined. Newly synthesized ionic
ruthenium based polypyridyl dye and polymer have been utilized to fabricate
light emitting devices with high brightness (ca. 100 cd/m 2 ) at voltages in the
range of 5 - 7 volts. Multilayers containing the synthesized ruthenium based
polypyridyl polymer produce highly efficient (ca. 2 % quantum efficiency) and
reversible electrochemiluminescent devices. The fabrication and device
evaluation of new heterostructure thin films based on this new light emitting
dye and polymer are presented. The bilayer thickness and composition as
well as the total thickness of the heterostructure strongly affect the functional
properties of these multilayer films.
Thesis Supervisor: Michael F. Rubner
Title : TDK Professor of Polymer Science
TABLE OF CONTENTS
.........................................
PageTitle
Title Page....................................................................1
Abstract...........................................
. . . .............................................................................. 2
Table of Contents..................................................
.....................................................
3
List of Figures .....................................................
.......................................................
8
List of T ables ................................................................................................................... 12
CHAPTER 1 : INTRODUCTION AND BACKGROUND
1. Introduction................................................................................................................
13
1.1 M otivation ................................................................................................. 13
1.2 Applications of Functional Organic Thin Film..................................
15
1.3 Sequential Layer-by-Layer Adsorption Technique ................................... 17
2. B ackground..............................................................
............................................. 21
2.1 Polyelectrolytes in Solution.....................................
........ 21
2.2 Interaction Forces between Polyelectrolyte and Substrate........................23
2.3 Structure of Adsorbed Polyelectrolyte .......................................... 24
2.4 Polyelectrolyte Adsorption..........................
......
................. 25
A. Polyelectrolyte Adsorption on Surface with Opposite Charges ....... 26
A.1 Strong Polyelectrolytes ......................................
... 26
A.2 Weak Polyelectrolytes ......................................
.... 27
B. Polyelectrolyte Adsorption on Uncharged Surfaces ....................... 29
C. Neutral Polymer Adsorption on Polyelectrolyte Deposited
Surface ............................................................
...........
.......... 30
2.5 Linear Growth of Sequentially Adsorbed Multilayer Film ....................... 31
2.6 Concentration of Dipping Solutions .......................................
..... 32
2.7 Multilayer Films of Polyelectrolytes and Small Dye Molecules ................. 32
REFER EN C E S .........................................................
.................................................. 35
CHAPTER 2: EXPERIMENTAL PROCEDURES
1. M aterials .................................................................................
............................. 39
1.1 Polyelectrolyte Solution Preparation..............................
.......
39
A. Poly(aniline) Solution.............................................39
B. Poly(thiophene-3-acetic acid) Solution.............................
C. Other Polyelectrolyte Solutions................................
.... 40
...... 40
1.2 Dye Solution Preparation ...................................................
40
1.3 Sulfonated tris(phenanthroline)ruthenium (II) dication complex,
[Ru(phen')3] + 2 Synthesis............................................
................... 41
1.4 Poly(tris(bipyridine)ruthenium(II)dication complex),
poly([Ru(bpy)3]+ 2 )ester Synthesis.........................................................
2. Substrate Preparation.................................................
42
.......................................... 43
2.1 Hydrophilic Slide Substrate ...........................................
.......
2.2 Hydrophobic Slide Substrate .........................................
........ 43
2.3 Positively Charged Slide Substrate .......................................
43
...... 43
3. Layer-by-Layer Sequential Adsorption Technique ........................................ 44
3.1 Hand Dipping Procedure ....................................................
44
3.2 Automatic Dipping Procedure ................................................. 44
4. Characterization Methods........................................................45
4.1 Contact Angle Measurement .........................................
4.2 Thickness Measurement............................................
A . Profilom etry .....................................................
........ 45
.................... 46
.........
.......... 47
B . Ellipsom etry....................................................................
4.3 Visible and Near ultraviolet Spectroscopy ......................................
47
48
4.4 Infrared Spectroscopy ......................................................
49
4.5 Fluorescence and Electroluminescence Spectroscopy ...............................
50
.......................... 51
5. Light Emitting Device Fabrication.................................................
5.1 Indium tin oxide (ITO) Substrate...............................
................................
5.2 Film Fabrication on ITO substrates................................
.......
51
51
5.3 Al Evaporation........................................................52
5.4 Current-Voltage and Light-Voltage Measurement ....................................
52
5.5 Device Lifetime Testing .....................................................
53
5.6 Efficiency of Device Performance..............................................................53
R E FEREN CE S .........................................................
.................................................. 55
CHAPTER 3 : SURFACE MODIFICATIONS AND LAYER STRUCTURES
OF LAYER-BY-LAYER SEQUENTIAL ADSORBED MULTILAYERS
1. Introduction.........................................................
................................................. 56
2. Contact Angle and Wettability...............................................
2.1 D efinitions ..................................................
...................... 58
............................................ 58
2.2 Sensitivity of Contact Angle Measurement............................................63
2.3 Surface Roughness Effect on Contact Angles ........................................ 63
3. Surface Modification by Layer-by-Layer Sequential Adsorption Technique ........... 64
3.1 Surface Properties of 1 Bilayer of Several Polyelectrolytes ......................
64
3.2 Surface Properties of PAH/PAA and PAH/PTAA Multilayer Films..........68
3.3 Acid-Base Interaction in Wetting...................................
........ 71
3.4 Fabrication of Completely Wettable Surface ......................................
4. PAH/PAA Multilayer Film Fabrication .......................
73
....... 75
5. Contact Angles of PAH/PAA multilayer films......................................80
5.1 Salt Effect....................................................
............................................
80
5.2 Solution pH Effect................................................................................. 82
5.3 Concentration Effect .......................................................
5.4 Infrared Spectra of PAH/PAA Films.................................
85
...... 86
5.5 Aging Behavior of PAH/PAA Multilayer Films .......................
88
6. D iscussion ..........................................................
.................................................. 90
7. C onclusion..........................................................
.................................................. 96
R E FE REN CE S ...............................................................................
............................ 97
CHAPTER 4: MULTILAYERS OF FUNCTIONAL DYE MOLECULES
AND POLYELECTROLYTES
1. Introduction........................................
........................................
100
2. Structural Aspects of Dye Multilayer Film.................................
101
2.1 Fluorescein Complexion Multilayer Film..................................
....
2.2 Bordeaux R and Crystal Violet Dyes...................................
...... 104
3. Functionalized Multilayer Film s....................................................................................
3.1 pH indicating Azo dye Films......................................
3.2 Porphyrin dye Multilayer Films ........................................
102
05
......... 105
...... 108
3.3 Infrared Absorbing Dye Multilayer Films....................................................12
4. Ru complex Multilayer Films .....................................
114
4.1 Sulfonated [Ru(phen')3] + 2 / Polyelectrolyte Multilayer Film .................. 116
4.2 poly([Ru(bpy)3] + 2 ) ester / Polyelectrolyte Multilayer Film .................... 117
4.3 Sulfonated [Ru(phen')3] +2 / Poly([Ru(bpy)3]+ 2 )ester Film ...................... 118
5. C onclusion.................................................................................................................. 120
REFEREN CES ...........................................................
. ...............................................
121
CHAPTER 5: NOVEL RUTHENIUM POLYPYRIDYL COMPLEXES FOR
LIGHT EMITTING DEVICES
1. Introduction..................................
......................... . . ............................................. 123
1.1 Organic Light Emitting Devices ........................................
....... 123
125
1.2 Structure of Organic LED s.........................................................................
1.3 Fabrication Methods for Organic LEDs .....................................
.... 127
2. Ru Polypyridyl Complexes as Light-Emitting Materials ..................................... 129
2.1 Characteristics of Ruthenium Polypyridyl Complexes...........................
2.2 Single Layer Devices of Sulfonated [Ru(phen')31+
2
129
and
Poly([Ru(bpy)3] +2 )ester .....................................
132
3. Sulfonated [Ru(phen')3] + 2 and Polyelectrolyte Multilayer Films .......................... 137
4. Poly([Ru(bpy)3]+ 2 )ester and Polyelectrolyte Multilayer Films ............................ 140
4.1 Poly([Ru(bpy)3]+ 2 )ester and Poly(acrylic acid) Devices...........................140
A . Charging Effect..............................................
..........
........... 143
B. Thickness-Dependence Behavior ................................... 145
4.2 Poly([Ru(bpy)3]+ 2 )ester and Sulfonated poly(styrene) Devices.................147
5. Poly([Ru(bpy)3]+ 2 )ester and Sulfonated [Ru(phen')3] +2 Multilayer Films......... 148
6. Disscussion of Ru complexes Based LEDs...............................
149
7. C onclusion .................................................................................................................. 15 1
REFER EN CE S .................................................................................................................
153
CHAPTER 6: CONCLUSION, SUMMARY, AND FUTURE WORK .................. 157
Acknowledgments ........................................
163
LISTS OF FIGURES
Chapter 1
Figure 1.1.
A simple model of the layer-by-layer sequential adsorption
technique. ........................................................................ ................... 18
Figure 1.2.
Examples of several materials for the layer-by-layer deposition
technique. ................................................................................................. 20
Figure 1.3.
Structure of an adsorbed polymer ......................................
....
25
Figure 2.1.
Sulfonated Ru(phen') 3+2Cl 2 (I) synthesis scheme ...............................
41
Figure 2.2.
Poly([Ru(bpy)3]+ 2)ester (II) synthesis scheme. .................................. 42
Figure 2.3.
Chemical structures of (a) hexamethyl disilazane (HMDS) and (b)
N-(2-aminoethyl-3-aminopropyl) trimethoxysilane (TMS).................44
Chapter 2
Chapter 3
Figure 3.1.
A sessile liquid drop on a flat surface indicating contact angle. ............. 59
Figure 3.2.
Chemical structures of polyelectrolytes used to construct
multilayer thin films .....................................................
65
Figure 3.3.
Contact angles of PAH/PTAA layers on hydrophilic slide glasses
and PAH/PAA layers on hydrophobic slide glasses ................................ 69
Figure 3.4.
Contact angles of (a) hydrophobic slide glass and (b) 1 bilayer of
PAH/PAA on hydrophobic slide glass. ................................................... 70
Figure 3.5.
Contact angle titrations of 3 bilayers of PAH/PAA with PAA as
outerm ost layers .......................................................................................
72
Figure 3.6.
Aging effect on contact angles of PAH/PAA layers in the air. ............. 74
Figure 3.7.
Aging effect on contact angles of PAH/PAA layers at 90 "C .................. 74
Figure 3.8.
Linear growth of several sequentially adsorbed PAH/PAA films. ......... 76
Figure 3.9.
The amount of methylene blue dye adsorption onto PAH/PAA
layers fabricated by pH 2.5, 10-2 M of PAH with MgC12 0.4 M
and pH 2.5, 10-2 M of PAA solutions.............................
..... 78
Figure 3.10.
Salt in PAH solution effect on contact angles of PAH/PAA layers
fabricated by pH 2.5, 10-2 M of solutions............................................ 81
Figure 3.11.
Contact angles of PAH/PAA layers fabricated by pH 4.5 of
10-2 M solutions .........................................................
82
Contact angles of PAH/PAA layers fabricated by pH 2.5, 10-2 M
of PAH and pH 4.5, 10-2 M of PAA solutions ....................................
83
Figure 3.12.
Figure 3.13.
Contact angles of PAH/PAA fabricated by pH 4.5, 10-2 M of PAH
and pH 2.5, 10-2 M of PAA solutions.............................
..... 84
Figure 3.14.
Contact angles of PAH/PAA fabricated by pH 2.5 of 10- 3 M PAH
and PAA solutions ...................................... ................
86
Figure 3.15.
Infrared spectra of PAH/PAA films on Znse. .....................................
87
Figure 3.16.
Aging effects on contact angles of PAH/PAA multilayer films
fabricated by pH 2.5 of 10-2 M PAH and PAA solutions ................... 89
Figure 3.17.
Aging effects on contact angles of PAH/PAA multilayer films
fabricated by pH 2.5, 10-2 M of PAH with MgCl 2 0.4 M and pH
2.5, 10-2 M of PAA.solutions....................... ...............
89
Figure 3.18.
A proposed model of the layer-by-layer deposition. ...........................
91
Figure 3.19.
Mechanism of PAH/PAA layer adsorption..........................
..... 93
Chapter 4
Figure 4.1.
Chemical structures of used polyelectrolytes ...................................... 102
Figure 4.2.
Chemical structure of fluorescent complexion dye. .............................. 102
Figure 4.3.
Linear deposition of fluorescein complexion/PAH film ........................
103
Figure 4.4.
Chemical structures of (a) Crystal Violet and (b) Bordeaux R..........
104
Figure 4.5.
Linear deposition of Bordeaux R and PAH film. ..................................
105
Figure 4.6.
Chemical structure of Congo red dye ......................................
105
Figure 4.7.
Amounts of UV/Vis absorption of Congo red dye/polyelectrolyte
films.......................................................................................................... 10 6
Figure 4.8.
pH color change of a PAH/Congo red dye multilayer film. ................... 107
Figure 4.9.
Chemical structures of porphyrin dyes ........................................ 108
Figure 4.10.
Layer-by-layer growth of a) a cationic porphyrin dye/PMA
multilayer and b) an anionic porphyrin dye/PEI multilayer ................ 109
Figure 4.11.
Chemical structure of tetraphenyl porphine tetrasulfonic acid,
tetrasodium salt .......................................
110
UV/Vis spectra of anionic porphyrin/PAH multilayer film (420 nm
peak) and with Iron in the ring (370 nm peak) ...................
111
Figure 4.13.
Chemical structure of infrared absorbing dye...................................
112
Figure 4.14.
Linear growth of infrared absorbing dye/PAH multilayer.................... 113
Figure 4.15.
UV/Vis spectra of a multilayer film containing 5 layers of Ponceau
S (510 nm) and Infrared dye 125 (810 nm) .....................................
Figure 4.12.
114
Figure 4.16.
The chemical structures of (a) [Ru(bpy)3] +2 and (b) sulfonated
[R u(phen')3] +2 ................................................. .. ... ... ... ... ....................... . 115
Figure 4.17.
UV/Vis absorption of the poly([Ru(bpy)31+ 2 )ester/PAA and the
poly([Ru(bpy)3]+ 2 )ester/SPS films (20 bilayers) ..................................
118
Figure 4.18.
The amount of Ru complex adsorption at pH 2.5 and pH 4.5 of
[Ru(phen') 3 ]+ 2 and poly([Ru(bpy)3]+ 2 )ester solutions .......................... 119
Figure 4.19.
UV/Vis absorbance of sulfonated [Ru(phen')3] + 2 and
poly([Ru(bpy)3]+ 2 )ester film s..............................................................120
Chapter 5
Figure 5.1.
Figure 5.2.
Absorbance and photoluminescence (PL) spectra of sulfonated
[Ru(phen') 3 ]+2 . . ............ .........................
130
Absorbance and photoluminescence (PL) spectra of
poly([Ru(bpy)3]+ 2 )ester..................
131
Figure 5.3.
Current-voltage and light-voltage behavior of spin-cast sulfonated
[Ru(phen') 3 ]+2 device .....................................
133
Figure 5.4.
Current-voltage and light-voltage behavior of spin-cast
poly([Ru(bpy)3]+ 2 )ester device............................................................133
Figure 5.5.
EL spectra of sulfonated [Ru(phen')3]+ 2 and
poly([Ru(bpy)3]+ 2 )ester.................
134
Figure 5.6.
Stability data of (a) [Ru(phen')3] + 2 single-layer device and (b)
poly([Ru(bpy)3]+ 2 )ester single-layer device at 6 V............................. 136
Figure 5.7.
Current-voltage and light-voltage curves of PEI/[Ru(phen')3] + 2
m ultilayer film ..........................................................................................
37
Figure 5.8.
Stability of 30 bilayer PEI/[Ru(phen')3] +2 LEDs..............................
138
Figure 5.9.
Current-voltage and light-voltage curves of PAH/[Ru(phen') 3 ]+ 2
multilayer film ........................................
139
Current-voltage and light-voltage curves of poly([Ru(bpy)3] + 2 )
ester/PAA multilayer film ......................................
141
Figure 5.10.
Figure 5.11.
Current-voltage and light-voltage curves of PAA/
poly([Ru(bpy)3]+ 2 )ester generated by applying forward bias first ....... 144
Figure 5.12.
Current-voltage and light-voltage curves of PAA/
poly([Ru(bpy)3]+ 2 )ester generated by applying reverse bias first..........144
Figure 5.13.
Thickness dependence on Light-voltage curves of
PAA/poly([Ru(bpy)3]+ 2 )ester. ............................................. 145
Figure 5.14.
Efficiency and maximum light output of 15, 25, 30, 35 bilayers of
PAA/poly([Ru(bpy)3]+ 2 )ester devices..............................
146
Figure 5.15.
Thickness dependence of poly([Ru(bpy)3]+ 2 )ester/SPS multilayer
dev ices ...................................................................................................... 14 8
Figure 5.16.
Current-voltage and light-voltage curves of poly([Ru(bpy)3] +2 )
ester/[Ru(phen')3] + 2 LED..............................................149
LIST OF TABLES
Chapter 3
Table 3.1. Different surface properties by changing polyanionic layers .................... 67
Table 3.2. Layer thickness of PAH/PAA with different solution conditions.............77
Table 3.3. UV/Vis intensity of methylene blue absorbance peaks (at 600 nm).............79
........ . . . . 88
Table 3.4. Relative amount of Infrared peaks at 1560 and 1700 cm - 1 ...............
Chapter 4
Table 4.1. Intensity of UV/Vis absorbance peaks of PAH/[Ru(phen') 3 ]+ 2 films ......... 116
Functional Multilayer Organic Thin Films Fabricated by Layerby-Layer Sequential Adsorption Technique
CHAPTER 1
INTRODUCTION AND BACKGROUND
1. Introduction
1.1
Motivation
Organic materials show considerable promise for such applications as
optoelectronics, molecular electronics, and bioengineering. Through relatively simple
chemistry, organic materials can be synthesized and then functionalized to give the system
its novel properties. Thin films containing the organic materials typically have a thickness
ranging from a few nanometers (i.e. a monolayer) to several thousand. Such organic thin
films are known to have great potential for integrated optics, sensors, friction-reducing
coatings, surface modification, biological substrates, and patternable materials [1.1 - 1.2].
One of the primary barriers to wide-spread use of organic thin films is a fabrication
method to build up individual organic molecules into uniform and heterostructure
architectures. The use of organic thin films has also been hindered by the presence of
chemical impurities and defects in the uniform film that adversely affect mechanical and
thermal properties as well as chemical stability. The investigation of a technique for
producing ordered heterostructures and limiting the impurities and the defects is very
important to the utilization of the organic materials.
Various attempts to make uniform multilayer organic thin films have been
investigated for several decades [1.3]. These include Langmuir-Blodgett deposition, spincasting, and chemisorption. The Langmuir-Blodgett method [1.3 - 1.6] is based on the use
of surface-active molecules called amphiphiles which do not dissolve in water but have
hydrophilic groups at the end. A well-organized monolayer film forms at the water
interface as the amphiphilic molecules attempt to minimize the repulsive forces between
water and the non-polar hydrophobic groups. Monolayers can be transferred onto a
substrate by dipping it into a Langmuir-Blodgett trough that contains the amphiphiles
floating on water. This technique has been used to form ordered multilayered films [1.7].
However, one problem associated with this technique is their weak adhesion and interactive
forces between layers. These weak interactions easily lead to misalignment of the layers
and unstable structures after long periods of time. Several other problems exist, including
limitations on functional groups that can be used and restrictions on substrate shape.
In comparison, spin-casting methods [1.8] are simple and widely applied
techniques for producing a single layer. With spin-casting, organic materials with a wide
variety of functional groups can be rapidly made. In general, small dye molecules are
mixed in a binding polymer solution to prohibit the crystallization of the small molecules in
the solid state. Unfortunately, this technique has not been used to fabricate ordered
multilayered structures.
Chemisorption methods [1.9 - 1.11] have been investigated as a way of building
multilayer films on various substrates. Successive chemical reactions graft each layer to
form multilayer films. Various functional materials are often incorporated into a multilayer
film with chemical reactions between layers. The problems in chemisorption methods lie in
that only specialized functional reactions are possible for the application and that some
difficulties occur in limiting the reaction only to the surface. It requires a reaction yield of
about 100 % in order to keep the same amount of material on the surface from layer to
layer.
These methods either have limitations in fabricating multilayer films or in building
up organized thin multilayer films. Our group and others recently developed a novel
"layer-by-layer sequential adsorption" technique, that overcomes many of the problems
associated with the aforementioned techniques. It uses ionic interactions to make multilayer
films [1.12 - 1.21].
In contrast to other organic thin film fabrication methods, the
sequential adsorption technique is a very versatile means of fabricating multilayer thin
films. There are several advantages that the sequential adsorption method offers. First, a
wide array of functional materials can be used. The only requirement is that the functional
polymers or dye molecules have at least two ionic charge groups. A second and very
important advantage is that the method has no substrate shape, size, or treatment
requirements. Therefore, this will make it possible to use multilayer organic thin films for
a much wider variety of applications than ever before.
This work has focused on several fundamental issues associated with this new
technique. The first part of this thesis examines the mechanism by which the adsorbed
multilayers modify the surface properties of various substrates. The second part of the
thesis shows the feasibility of using small functional dye molecules in the adsorbed film.
Finally, the third section describes the fabrication of electrical and optical devices
containing a ruthenium polypyridyl complex based dye and polymer molecules.
1.2
Applications of Functional Organic Thin Film
As mentioned earlier, organic materials have been developed for electrical,
biological, and optoelectrical applications. In the future, successful application of these
materials in electrical and optical devices and replacements of sensor components will
require uniform and ordered films. Precise designs and arrays of organic layers will be
necessary for producing reliable high-quality devices. Some of the most promising
applications [1.1 - 1.3, 1.22] can be summarized as follows:
1) Organic based displays: Organic based light-emitting devices have been of considerable
interest for use in large area displays. These devices have a wide color range from red to
blue. Processing methods are extremely important in order to produce uniform devices. At
this point, liquid crystal displays using organic materials as passive polarizers are the type
of material used for this particular application.
2) Patternability of thin films: Organic thin films have also been used extensively for
development of photoresist patterns. As an easier way is developed to produce thinner and
more uniform patterns, we could fabricate dense and more complicated circuits. The use of
organic films in recording media is also an attractive project. Several different light
responsive organic materials are being considered for this application because of their
higher storage density compared with magnetic recording devices.
3) Packaging and insulating layers for integrated circuits: Insulating layers (i.e., epoxy
resin, polyimide resin) are sealed on top of a silicon crystal surface and packaged with a
semiconductor chip with attached electrical connections. As chip designs become smaller
and more complicated, organic thin films for faster dissipation of heat and lower thermal
expansion are required.
4) Surface Functionalization: Adhesion, wetting, friction, biocompatibility, and chemical
reactivity are dramatically altered by the attachment of one or more layers of organic
functional groups at the material surface. Vastly different functional groups such as -SO3-,
-NO 2 , -CF 3 , -CO 2 H, and -OH, can be grafted and adsorbed onto various substrates for a
wide variety of applications. Since organic polymers generally have low surface energies
and are rather stable in air, they are often used for protective coatings. The highly tailorable
functionality that polymers can have then makes it possible to develop coatings for different
applications.
5) Sensor and optic application: Chemical response, photoelectronic, piezoelectrolic, and
electrochemical materials have been investigated for sensor applications.
Uniform
multilayer film is important for their functional behavior. Impurities and defects in the
uniform films are detrimental for these applications. In thin-film optics, nonlinear optical
effects (second and third harmonic generation) are of particular interest. In organic
materials, the nonlinear effects depend on the intrinsic polarizability of the molecules as
well as the ordering of the organic materials.
The next section will briefly describe the unique features of the sequential layerby-layer adsorption technique.
1.3
Sequential Layer-by-Layer Adsorption Technique
Recently, Decher and co-workers introduced a novel technique known as "layer-
by-layer sequential adsorption" [For review papers on the technique, see 1.23 - 1.24].
This technique is based on the spontaneous adsorption of polyelectrolytes onto an
oppositely charged surface. A simplified model of the technique is depicted in Figure 1.1.
The electrostatic interactions are what hold the layers together and provide stable strong
interaction. To build a multilayer structure, sequential dippings are performed using
positively and negatively charged chemical species in alternating layers. The substrate is
generally charged with either silane species or cationic polyelectrolytes. This makes it
possible to adsorb polyions from a solution of oppositely charged (i.e., anionic) molecules.
Since the process is done by dipping a substrate in solutions, there are no size or
shape limitations. After the first layer is adsorbed, the sample is rinsed several times and
then immersed in the oppositely charged solution. This procedure is then repeated until the
required number of layers have been deposited. One of the remarkable properties of this
technique is the feasibility of controlling the thickness and the structure of a layer through
changes in pH, salt quantity, and polyelectrolyte concentration.
The thickness of a
deposited layer can be controlled from a few
A to several
10-A units by changing the
conditions of the solution. These effects will be discussed in the section 2.
4
-
0-
D
Figure 1.1. A simple model of the layer-by-layer sequential adsorption technique.
Another advantage of this technique is the precise control that it offers. By simply
changing the order in which the layers are applied, the multilayered structures can be
controlled in a precise way. This technique provides a way to create new thin film
heterostructures with novel optical, electrical, and biological properties. For example, our
group have recently demonstrated that such layer-by-layer deposited multilayers can be
used to create semi-transparent and electrically conducting coatings as well as thin film light
emitting diodes [1.17 - 20, 1.25 - 1.27]. The heterostructures of poly(p - phenylene
vinylene) (PPV) light emitting devices [1.25] are an interesting case because their
properties are affected by the surrounding layers. By changing the polyelectrolyte layers
and the order of the layers, the device showed the different performances.
One important aspect of this method is the deposition speed, which is relatively
fast. Polyelectrolytes have very steep isotherms and reach the plateau region within a few
minutes. This is a result of a high affinity type adsorption of polyelectrolytes. The
adsorption process that occurs at the interface between the deposited polyelectrolyte and the
counter charged polyelectrolytes in solutions may be producing kinetically determined
structures [1.28, 1.29].
The sequential adsorption process can be used with many different materials.
Decher et al [1.12 - 1.13] first used this technique with a combination of a polyelectrolyte
and a bolaform material (Figure 1.2). The latter is a rigid rod molecule with two cationic or
anionic functional end groups. Several researchers expanded upon this technique by
incorporating various polyelectrolytes such as poly(allylamine), poly(acrylic acid),
sulfonated poly(styrene), poly(methacrylic acid) in the multilayers [1.30]. Recently, our
group succeeded in forming multilayered structures based on conducting polymers such as
poly(aniline), and poly(thiophene-3-acetic acid) [1.17 - 1.19]. The conductivity of these
films can be as high as 40 S cm - 1 . Multilayered structures based on in-situ polymerization
of poly(pyrrole) were also developed by our group [1.32]. Another important achievement
in our group has been the incorporation of various precursor types of polymers [1.25,
1.33]. The precursor form of polymers, such as PPV and poly(imide) were multilayered
with oppositely charged polyelectrolytes. After the multilayer film was fabricated, the
precursor layers were converted to PPV or poly(imide) layer. Fullerenes ("Bucky Ball")
fitted with ionizable surface groups were formed to multilayer films with the help of several
polyelectrolytes. Other groups introduced the technique for the biological application.
Proteins or polypeptides were ionically layer-by-layer deposited to form multiprotein
systems [1.34,1.35]. The multilayer film of polyelectrolytes and DNA were reported for
possible applications in ecological system. Simple charged metal particles were also
recently reported to be incorporated into a multilayer film [1.36, 1.37]. Figure 1.2
a) Molecules with extended chains (Bolaform)
H
H
I
I
/
"
0
*
0
%O%%4+
0
3b)
Polyelectrolytes
b) Polyelectrolytes
-vWCH 2 -CHem
I
w•CH 2 -CH 2 - NH 2 ""
+
CH 3
~CH2 -CHmm
1
*wCH2 --Cw
I
COO
CH 2
-
coo
NH
+ 3
c) Conducting polymers
H
N--M
CH2 COO
d) Precursor polymers
I
3-0
-
.
s
+j=
AA
I
HO
II
L;-N
0 H --\ ~
' L)
V
Figure 1.2. Examples of several materials for the layer-by-layer deposition technique.
summarizes several different materials applied for the layer-by-layer deposition technique.
I expanded this technique to form layer-by-layer deposited multilayer films using
small dye molecules, which have only a few charges in the chemical structure. Several
different examples of the multilayer films of functional dye molecules and polyelectrolytes
will be discussed in chapter 4.
2.
Background
Understanding polyelectrolyte behavior in solution and adsorption phenomena at
interfaces is important for obtaining precise control of multilayer structures. Several
aspects of this new technique will be described in the following sections.
2.1
Polyelectrolytes in Solution
When a polymer dissolves in a solvent, the conformation of the polymer chain and
the associated energy states are determined by the nature of the polymer and the quality of
the solvent. The end-to-end distance, or the radius of gyration, is generally used to
characterize the chain conformation in solution. For 'good' solvents, the radius of gyration
would be larger than for 'poor' solvents. Solvent quality can also be expressed as the size
of the excluded volume of the polymer chain. This quantity refers to the effective size of a
monomer or unit size of a polymer chain in solution [1.38].
In a good solvent, the polymer chains expand to increase contacts with the solvent
and adopt a self-avoiding random walk. As the solvent becomes poor, we reach the theta
state, where the excluded volume decreases to zero. In this case, the statistical behavior of
the polymer becomes a random walk as opposed to a self-avoiding one. As the quality of
the solvent decreases below the theta state, the chains attempt to exclude the solvent in
order to increase polymer-polymer interactions. In concentrated solutions where the
solvent is a poor one, the polymer will eventually precipitate. A polymer in poor solvent
has more tendency to adsorb onto a surface. This indicates that polymer adsorption would
be much more favorable in poor solvents due to the instability of the polymer in the
solution.
A polyelectrolyte is defined as a polymer that has many ionizable groups either on
the main chain or in pendant groups. The strength of the ionic charge groups determines
the strength of the polyelectrolyte. If the polyelectrolyte is a strong one, then pH of the
solution will not change the nature of the charged species. On the other hand, if the
polyelectrolyte is a weak one, then a change of pH can affect the charge density of the
polyelectrolyte. This is an important issue because the interaction of a polyvalent macroion
and counter ion is the source of repulsive forces that give polyelectrolytes some of their
unique characteristics.
The number of dissociated groups or repulsive forces between charge groups will
depend on the amount of salt in the solution as well as pH of the solution. In weak
polyelectrolytes, such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA), the
degree of dissociation is highly dependent on the pH of the solution. The dissociation
constant ( pKa) of the electrolyte groups represents the equilibrium between an acid and a
conjugated base. At pH below pKa, the low degree of dissociation of charge groups in
PAA and PMA makes the chain take on a coiled conformation. At pH above pK,, the
system may take on an extended chain conformation as a result of the repulsive forces
between charges. When this happens, the polyelectrolyte solubility in water is also
increased due to a result of the ionization of charge groups on the polymer. According to
Oosawa [1.39], poly(acrylic acid) with a degree of polymerization= 1000 forms a spherical
random coil with a radius of gyration of 200
radius of gyration increases up to 2500
A.
A at a low pH.
However, at a high pH, the
The free acid form (-COOH) decreases the
solubility of PAA and makes it less soluble in water. The carboxylate group (-COO-) on
the other hand increases the solubility in water and makes the system behave like a polymer
in good solvent. The following equation shows the relationship between pH, pKa and
degree of dissociation, a [1.40].
(1.1)
pH = pKa + log(a / (1- a))+ f(I)
where f(I) is small correction factor depending on the ionic strength (I). a is the ratio of
the base forms (dissociated forms) to number of electrolyte groups. If pH is low, a will
decrease which means that most of the electrolyte groups will remain in a neutral state. If
pH is higher than pK,~, a would increase with this high charge group population, the
repulsion forces make polyelectrolytes take on extended chain conformations. Addition of
salt will also change the behavior of the polyelectrolyte, even at high pH values. The
bound counterions of the salt will shield the charge of the polyelectrolyte and make it take
on a coiled conformation due to the apparent reduction in repulsive forces. It is in this way
that chain conformations of polyelectrolytes are strongly affected by the conditions of the
solutions.
2.2
Interaction Forces between Polyelectrolyte and Substrate
After a polymer adsorbs onto a substrate, there are fewer possible conformational
states. As a result, the entropy decreases when the molecules are adsorbed onto a
substrate. The overall entropy change at the adsorption is the negative value of AS.
AG = AH - TAS
(1.2)
Through the fundamental equation of thermodynamics, a spontaneous reaction
occurs when AG is negative. Only in cases when the strength of the enthalpic term of the
free energy expression has a larger value to overcome the positive value of the entropy
term, then adsorption will be the spontaneous reaction. During adsorption, if the polymer
decreases the value of the entropy term, or increases enthalpy term of the interaction
strength, then deposition of the chain onto the surface is favored. [1.41 - 1.42].
The interaction between substrates and polyelectrolytes is the major factor to
consider because the entropic term is generally always negative and does not change
appreciably as the chemistry of the system is changed. In the sequential adsorption
technique, the ionic interaction forces between a substrate and the polyelectrolytes are
strong enough to cause adsorption to occur. The strength of these binding forces are
enhanced by the multiple sites of charges along the polyelectrolyte. Even if the segmental
binding energy is very small, the large number of segments along a polymer chain lead to a
strong overall binding force. Even certain types of uncharged polymers can adsorb onto a
variety of surfaces. This happens because most polymers have a large number of either
electron donating or electron withdrawing groups. Polymer adsorption is defined as a high
affinity between a polymer and a surface. The poor solvent also facilitates polymer
adsorption, but the attractive forces at interface still dominate the adsorption.
According to the result reported by Fowkes [1.43], the acidic polymer (poly(vinyl
chloride)) only adsorbed onto the basic substrate (calcium carbonate) and did not adsorb
onto the acidic substrate (silica). Whereas the basic polymer (poly(methyl methacrylate))
only adsorbed onto the acidic substrate (silica). These results proved that the interaction
forces at interfaces are important sources of polymer adsorption. The strength of surface
interactions is crucial for the polymer adsorption.
2.3
Structure of Adsorbed Polyelectrolyte
The rate of polyelectrolyte adsorption is so fast that the morphology of
polyelectrolyte layer is kinetically determined at the first stage of adsorption [1.41 - 1.42].
The number of bound and unbound sites is determined by the dipping conditions of
polyelectrolyte solutions. In the sequential adsorption, it can be seen that the unbounded
charges of the polyelectrolytes are dangled in the polyelectrolyte solution and ready to work
as interaction sites for the additional layer of oppositely charged polyelectrolytes. The
structures of adsorbed polymer chains on the solid surface are characterized by trains,
loops, and tails (see Figure 1.3). The parts of trains of the adsorbed polyelectrolytes work
as bonding forces to hold the adjacent polyelectrolytes. The unbounded charge groups are
interacting with the oppositely charged polyelectrolytes in the consecutive dipping solution
and make a binding to hold the consecutive layer.
Tail
Loop
Trainn
X~
o
Tail
Figure 1.3. Structure of an adsorbed polymer.
The highly charged polyelectrolyte makes thin and tail layer structures, while the
less charged polyelectrolyte does thick and loopy layer structures. The number of charge
groups on a surface, solvent properties, and the number of charges on the polyelectrolyte
are important in determining the structure of a polyelectrolyte.
2.4.
Polyelectrolytes Adsorption
Polyelectrolyte adsorption is a complex process with many components such as
the interactions among solvent, surface, salt and polyelectrolytes [1.44 - 1.45]. The
surface charge density as, the number of charges in the polyelectrolyte qm, and the ionic
strength Cs are three main factors that affect polyelectrolyte adsorption. The pH of the
dipping solution changes the degree of ionization of electrolyte groups in polyelectrolytes
and the charge density of a substrate or a deposited layer. Monovalent or divalent salts,
such as NaC1, MgC12 and MnCl2 are dissolved in the polyelectrolyte solution to increase
ionic strength. The salts work to screen the charges on a substrate and polyelectrolyte. In
the layer-by-layer adsorption technique, the number of charge sites of a polyelectrolyte and
a surface determine the strength of the interactions and the amount of adsorption. The
strong interaction between layers is required to maintain multilayers that do not dissolve in
the oppositely charged solution.
A.
Polyelectrolyte Adsorption on Surface with Opposite Charges
As discussed in the previous section, electrostatic forces drive polyelectrolyte
adsorption. The strong interaction between a polyelectrolyte and a substrate is responsible
for the rapid adsorption and the steep isotherm.
When strong acids or bases are
incorporated into the system, most of the functional groups are ionized for a wide range of
pH values. However, when weak acids and bases are used, the ionization of charge
groups is strongly dependent on the pH of the solution. Both cases will be discussed in
detail in subsequent sections.
A.1
Strong Polyelectrolytes
In this section, it is assumed that the charge groups of polyelectrolytes are fully
ionized at the pH level of the dipping solution. In this case of high charge density in the
polyelectrolyte, the electrostatic forces are the strong driving force for adsorption. As the
density of surface charges increases, the amount of polyelectrolyte adsorption also
increases to balance the surface charge. The increase, however, is not linear because the
accumulation of like charges on the surface hinders additional adsorption.
When salt dissolves into the solution, i.e., ionic strength is enhanced, two
opposing forces are introduced.
The ionic repulsions between the deposited
polyelectrolytes and those still in solution as well as the intrasegmental repulsions within a
polymer chain can be reduced. In addition, the counterions of the salts can shield the
charges on the surface and reduce the interaction forces at the surface. However, the
interaction forces at interface are still strong enough to overcome the screened charges on a
surface.
The enhanced ionic strength that the salt provided was responsible for increasing
the thickness of the adsorbed layers. The charge screening is interesting from a structural
point of view because this process can increase the number of loops and tails in the
structure of the adsorbed layers. The charge screening at the interface also decreases the
repulsive forces between the same charged polyelectrolytes in the solution and on the
surface. The additional polyelectrolytes are facilitated to attach to the surface. Several
research groups have reported the use of salts to produce thick layers [1.44 - 1.47]. At low
salt concentration, the polymer adsorbs as an extended structure due to the strong repulsive
forces.
In highly charged polyelectrolytes, molecular weight does not change the
thickness of adsorbed layers. Stockton [1.48] studied the molecular weight dependence
using the strong polyelectrolyte of poly(sulfonated styrene) (SPS) by changing the
molecular weight of SPS from 5 to 990k. The thickness per bilayer was not varied to the
different molecular weight of SPS. The highly charged SPS formed thin and train
structures where the long chain or the short chain is fully extended.
In conclusion, the charge compensation is the principal factor in binding the strong
polyelectrolytes to the oppositely charged substrate. The strong dependence of the ionic
strength was observed with the strong polyelectrolytes.
A.2
Weak Polyelectrolytes
The charge dissociation of the weak polyelectrolytes is a function of the local pH.
Not only are charges of already adsorbed weak polyelectrolytes affected but so are the
charges of polyelectrolytes in solution. The interaction forces at the interface are also rather
weak, but still sufficient for adsorption to occur. Adsorption behavior of weak acid
polyelectrolyte can be classified into two groups.
Those where the pH of the
polyelectrolyte solution is above the pKa (acid dissociation constant) of the polyelectrolyte
and those where it is below the pKa .
In the former case, the fully charged
polyelectrolytes, which are similar to strong polyelectrolytes, have strong electrostatic
forces at the interface. In addition, the amount of adsorption is limited by the repulsive
forces at the interface hindering a deposition of like charges. In the latter case where the
pH is about or lower than the pKa, the intrasegmental and interfacial repulsion forces both
decrease. This is a result of the low fraction of charge segments on the polyelectrolytes.
Both theoretical [1.49] and experimental results [1.50, 1.51] show that the maximum
adsorption of weak polyelectrolytes is obtained at a pH that is lower than the pKa by 1 or 2
units. For example, the carboxylic acid of poly(acrylic acid) or poly(methacylic acid) is a
weak acid with an pKa around 4.0. When pH is much lower than the pKa, the neutral
carboxylic acid groups are not attracted to the charged surfaces as strongly. When pH is
much higher than the pKa, the strong electrostatic forces cause the polyelectrolyte
adsorption with the extended chain form. In lightly charged weak polyelectrolyte systems,
layer thickness depends on molecular weight due to the increase in the probability of
forming loop and tail structures. The pH of a weak polyelectrolyte solution greatly
influences the thickness of the adsorbed layers.
Ionic strength in a weak polyelectrolyte solution affects the adsorption behavior in
different ways according to the charge density of the weak polyelectrolyte. In the low
charge density of a polyelectrolyte, the ionic strength screens the low fraction of charges
and thereby the attraction force decreases. The adsorbate (deposited polyelectrolytes) is
nearly neutral. Sometimes, polyelectrolyte adsorption does not occur. At the highly
charged state, the ionic strength screens the repulsive forces of charge segments in the
polyelectrolyte. The decreased intrasegmental repulsion enhanced the polyelectrolyte
adsorption with tails and loops. The adsorbed fully charged polyelectrolytes are going to
keep some net charges on the surface.
In the layer-by-layer adsorption, it is expected that the charge density of an
adsorbed layer of weak polyelectrolytes varies with the pH of the dipping solutions. The
high pH of a dipping solution will increase the negative charges of poly(acrylic acid)
outermost layer, while the low pH of a dipping solution will decrease the charges. The
difference of charge density will affect the additional layer adsorption.
In summary, in a weak polyelectrolyte adsorption, pH and ionic strength will
change the surface charge density as well as the repulsive forces in the polyelectrolyte.
B.
Polyelectrolyte Adsorption on Uncharged Surface
Uncharged surfaces are frequently used as a substrate in the layer-by-layer
deposition. The interaction between an uncharged surface and a polyelectrolyte is much
weaker than the static ionic interaction at the charged surfaces. The interaction between
solvent-polyelectrolyte and polyelectrolyte-surface determine the adsorption behavior of
polyelectrolytes at an uncharged surface. When the strength of polyelectrolyte-surface
interaction exceeds the strength of solvent-polyelectrolyte interaction, the polyelectrolyte
deposits onto the surface. The rate of the adsorption is determined by the difference of two
interaction strengths.
The electrostatic effect in this case is that the segmental repulsion between the
same charges of a polyelectrolyte and electroneutrality of charge segment groups on a
surface. In general, with increasing charge density in polyelectrolytes, both the interactions
between solvent and polyelectrolytes and between polyelectrolytes and surfaces are
enhanced. If the surface has some of electron donor or acceptor groups, the level of
dissociation of electrolyte groups will enhance the polyelectrolyte adsorption.
In solutions of high salt content, i.e., ionic strength, the counterions decrease the
intrasegment repulsion and electroneutrality was easily achieved at the interface. The fixed
counterion to the polyelectrolyte segments will be freed at the polyelectrolyte adsorption.
The polyelectrolyte tends to adsorb to most surfaces with the coiled structure at high salt
content.
C.
Neutral Polymer Adsorption on Polyelectrolyte Deposited Surface
The polymer adsorption on charged surfaces is very similar to polymer adsorption
onto the uncharged surface except the adsorption dependence on salt in the solution. Most
polymers, which have electron donor or acceptor groups, adsorb from solution onto a
surface. Even though a polymer loses some of its freedom through adsorption, most of
polymers gain a sufficient adsorption free energy per segment to overcome a certain
minimum adsorption free energy.
The solvency parameter, as can be expressed for Flory-Huggins interaction
parameter X [1.52], is important to polymer adsorption. The parameter X is defined as
X
=
z(hk2 -
2hll -
h2k2) /
kT
(1.3)
where z is coordination number and component 1 is a solvent and component 2 is
a polymer. The h means the interaction enthalpy per contact. If X is bigger than 0, the
solvent becomes poor and the polymer prefers the contact of similar segments. The amount
of the polymer adsorption is controllable by selecting the solvent.
The adsorption phenomena can be described as the exchange process upon
adsorption of a segment, where a solvent-surface interaction is displaced by a segmentsurface interaction. Through the same concept in the solvency parameter, Silberberg [1.53]
introduced the adsorption enthalpy parameter Xs which is defined as
s,= z'(hsl - hs2 +
h22 -~ 2
1)
/ kT
(1.4)
where z'is contact sites at the surface and component s is the surface. If X, is positive, the
adsorption is preferred. Those two parameters are the most important considerations in the
polymer adsorption.
The adsorption, in general, increases with decreasing solvency parameters, while
with increasing the adsorption enthalpy parameters. The polymers in good solvents require
strong interaction for the deposition onto the surface. The poor solvent enhanced the
adsorption of polymer onto the surface. The higher molecular weight polymer has stronger
tendency to adsorb onto the surface because the numbers of sites decreases the solubility of
polymer. This explained why polymers are easier to adsorb onto the surface, compared
with the small molecule adsorption where the entropy penalty is much lower. The polymer
adsorption on charged surfaces follows the general trends of polymer adsorption with
enhanced interaction forces. The ionic strength again does not influence the polymer and
may seal the charges on the surface to lose the interaction forces.
The polymer adsorption is complex and is varied to the conditions of number of
charges, ionic strength and surface properties. By considering several parameters, we can
find the right condition to make linear growth multilayer films and tune an adsorbed layer
thickness in the layer-by-layer deposition.
2.5
Linear Growth of Sequentially Adsorbed Multilayer Film
The adsorbed polymer chain links to substrates by several bound segments (n).
The Boltzmann factor for desorbed states can be expressed by exp(-nEb/KT), where the Eb
is the energy barrier of each segment desorption. The probability of desorption decreases
as the numbers of interaction segments increase. Compared with the small molecular
deposition, once deposited polymer is not likely to be depleted into the solution due to the
several interactions. The unbound charges of the deposited layer work as interaction sites
in the oppositely charged dipping solution. Once the dipping procedure keeps the same
charge density and the same structure of the multilayered films, the amount of adsorption
becomes constant and hereby the thickness increment of bilayer is sustained.
The nonlinear growth is sometimes observed in the cases where the surface
charges shrink or grow as the number of layer deposition increases. Most of nonlinear
adsorption is overcome by changing the dipping conditions to keep adsorbed layers from
desorbing, but some materials eventually lead to non-linear growth. For example, a highly
branched polyelectrolyte like poly(ethyleneimine) would increase the charge density
through the increased roughness and polyions with low charge density would lower the
available charges on a surface as the number of layers increase.
2.6
Concentration of Dipping Solutions
The concentrations of polyelectrolytes in this study are maintained in the 10-2 to
10-4 M of repeat unit concentrations.
This regime is still dilute solutions where
polyelectrolytes do not overlap each other. In general, higher concentration can produce
thicker layers in a given time due to a higher diffusion rate and concentration at the
interface. The dependence of overall thickness on concentration is affected by the affinity
type of the interface. A strong polyelectrolyte is less sensitive to the concentration of
dipping solutions than an uncharged polymer or a weak polyelectrolyte. In the case of
strong polyelectrolytes, the rate of adsorption is fast and covers the whole surface in the
early stage of the adsorption with mostly train structure.
The already adsorbed
polyelectrolytes limit the additional adsorption of polyelectrolytes by static ionic repulsions.
The tails and loops of the weak adsorbed polyelectrolytes are enhanced with the higher
concentration of polyelectrolytes at interface.
2.7
Multilayer Films of Polyelectrolytes and Small Dye Molecules
Through this study, this novel layer-by-layer adsorption technique was expanded to
build-up multilayer films of functional dye molecules and polyelectrolytes. Organic dyes
have shown many interesting properties for the possible applications in electronics and
optics [1.54]. The organic dyes, however, are hard to be used as a definite layer or
multilayer structures with other functional materials. In general, the dye molecules are
mixed with polymer binders and spun onto the substrates. The layer-by-layer adsorption
technique may be a unique way to fabricate the heterostructure of an organic dye monolayer
with other functional layers.
There are various differences between the adsorption of small dye molecules and
polymers, in many aspects. The enthalpy penalty upon the adsorption in polymer is much
greater than the small dye molecule upon adsorption, because no conformation change
occurs at the small dye molecule adsorption. The small dye molecules are willing to be
adsorbed onto the charged surface as long as the interactions exist. The adsorbed small dye
molecules, however, have much higher possibilities to be depleted from the surface,
because the small dye molecules have only a few charge groups bound to the layers. To
build up a multilayer structure, the stronger bonds are required for the adsorption of the
small dye molecules.
The organic materials for forming the individual layers in the layer-by-layer
deposition on the modified surfaces are either monomeric substances having more than two
ionizable functional groups of the same charge, or so-called polyelectrolyte type. Two
separate charges in the dye structures are required at least to incorporate dye molecules in
the multilayer structures. Unless there are sufficient interactions between dye molecules
and polyelectrolytes on the surface, multilayer films cannot successfully build up. The
different adsorption mechanism and layer structure could be determined by the kinds of
charges in the dye structures. For example, the case of strong acid like -SO3-, is used for a
wide range of pH of the dipping solutions and induce stronger bonds, but the weak acid
like -COOH, has small pH ranges for the processing to hold the polyelectrolyte to build up
multilayer films. The tails and loops of adsorbed polyelectrolytes work as binding sites
and deter the desorption of once adsorbed dye molecules. The loopy structure of layers
enhances the adsorption of dye molecules in the solution.
Up to this point, several issues in the sequential layer-by-layer deposition technique
were discussed. The technique employs several complex phenomena involved in polymer
adsorption and gives the possibility to fine tune the molecular structures and adsorption
behavior by changing pH, concentration and ionic strength of the polyelectrolyte dipping
solution. Various kinds of materials which have some charge groups can be used to build
up layer-by-layer multilayer films.
REFERENCES
[1.1]
Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.;
Israelachivile, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.
Yu, H.; Langmuir 1987, 3, 932.
[1.2]
Adv. Mater. 1991, 3, Special Issue on Organic Thin Films.
[1.3]
Ulman, A. "An Introduction to Ultrathin Organic Films from Langmuir-Blodgett
to Self-Assembly"; Academic Press: San Diego, 1991.
[1.4]
Blodgett, K. B.; Langmuir, I. Phy. Rev. 1937, 57, 964.
[1.5]
Roberts, G. "Langmuir-Blodett Films"; Plenum Press: New York, 1990.
[1.6]
Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92.
[1.7]
Kuhn, H.; MObius, D.; Bticher, H. in "Techniques of Chemistry, Part 3B, Vol.
1", Weissberger, A. and Rossiter, B. W. Eds.; Wiley: New York, 1973, p5 7 7 .
[1.8]
Spangler, L. L.; Torkelson, J. M.; Scot Royal, J. Poly. Eng. and Sci. 1990, 30,
644.
[1.9]
Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101.
[1.10] Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.; Marks,
T. J.; Lin, W.; Wong, W. L. Langmuir 1993, 9, 388.
[1.11] Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am.
Chem. Soc. 1994, 116, 295.
[1.12] Decher, G.; Hong, J. D. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430.
[1.13] Decher, G.; Hong, J. D. Thin Solid Films 1992, 210/211, 831.
[1.14] Lvov, Y.; Decher, G.; M6hward, H. Langmuir 1993, 9, 520.
[1.15] Lvov, Y.; Hass, H.; Decher, G.; M~ihwald, H.; Kalachev A. J. Phys. Chem.
1993, 97, 12835.
[1.16] Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772.
[1.17] Cheung, J. H.; Fou, A. C.; Ferreira, M.; Rubner, M. F. Polymer Preprints
1993, 34, 757.
[1.18] Cheung, J. H.; Fou, A. C.; Rubner, M. F. Thin Solid Films 1994, 244, 985.
[1.19] Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806.
[1.20] Fou, A. C.; Ellis, D.; Ferreira, M.; Rubner, M. F. Polymer preprints 1994, 35,
221.
[1.21] Yoo, D.; Lee, J-K.; Rubner, M. F. Mat. Res. Soc. Symp. Proc. 1996, 413,
395.
[1.22] Mat. Res. Soc. Bull. 1995, 20, Special Issue on Organic Thin Films.
[1.23] Knoll, W. Current Opinion in Colloid & Interface Sci. 1996, 1, 137.
[1.24] Decher, G. in "Comprehensive Suptramolecular Chemistry Vol. 9"; Sauvage, J.P and Hosseini, M. W. Eds.; Pergamon: Oxford, 1996, p507.
[1.25] Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl.
Phys. 1996, 79, 1.
[1.26] Tian, J.; Wu, C-C.; Thompson, M. E.; Sturm, J. C.; Register, R. A.; Marsella,
M. J.; Swager, T. M. Adv. Mater. 1995, 7, 395.
[1.27] Hong, H.; Davidov, D.; Avany Y.; Chayet H.; Faraggi, E. Z.; Neumann, R.
Adv. Mater. 1995, 7, 846.
[1.28] Papenhuijzen, J.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci.
1985, 104, 553.
[1.29] Meadows, J.; Williams, P. A.; Garvey, M. J.; Harrop, R. A.; Phillips, G. O.
Colloids and Surfaces 1988, 32, 275.
[1.30] Yoo, D.; Rubner, M. F. SPE Proceedings,Boston, MA, 1995, 2568.
[1.31] Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107.
[1.32] Fou, A. F.; Rubner, M. F. Macromolecules 1995, 28, 7115.
[1.33] Baur, J. : Rubner, M. F. To be submitted.
[1.34] Lvov, Y.; Decher, G.; Sukhorukov, G.; Macromolecules 1993, 26, 20.
[1.35] Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117,
22.
[1.36] Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370.
[1.37] Gao, M.; Zhang, X.; Yang, Y.; Yang, B.; Shen, J. Chem. Soc. Chem. Comm.
1994, 2777.
[1.38] Hiemenz, P. C. "Polymer Chemistry"; Marcel Dekker: New York, 1984.
[1.39] Oosawa, F. "Polyelectrolytes"; Marcel Dekker: New York, 1971.
[1.40] Mandel, M. in "Encyclopedia of Polymer Science and Engineering, 2nd. ed. Vol.
11."; John Wiley & Sons: New York, 1985, p7 3 9 .
[1.41] Fleer, G. J.; Lyklema, J. in "Adsorption from Solutions at the Solid/Liquid
Interface"; Parfitt, G. D., Rochester, C. H., Eds.; Academicn Press: London, 1983, p 1 5 3
[1.42] Silberberg, A. in "Encyclopedia of Polymer Science and Engineering, 2nd. ed.
Vol. 1"; John Wiley & Sons: New York, 1985, p577.
[1.43] Fowkes, F. M.; Mostafe, M. A. IEC Prod. R&D 1978, 17, 3.
[1.44] Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.;
Vincent, B. "Polymers at Interfaces"; Chapman & Hall: London, 1993.
[1.45] Van de Steeg, H. G. M.; Cohen Stuart, M. A.; Keizer, A.; Bilsterbosch, B. H.
Langmuir 1992, 8, 2538.
[1.46] Cosgrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986, 111,
409.
[1.47] Decher, G.; Lvov, Y.; Schimitt, J. Thin solid Films 1994, 244, 772.
[1.48] Stockton, W. B. Ph. D. Thesis, M.I.T., 1995.
[1.49] Bihmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990,
23, 2288.
[1.50] Blaakmeer, J.; BWhmer, M. R.; Cohen Stuart, M. A.; Fleer, G. J.
Macromolecules 1990, 23, 2301.
[1.51] Robb, I. D.; Sharple, M. J. Colloid Interface Sci. 1982, 89, 301.
[1.52] Flory, P. J. "Polymer Chemistry"; Cornell University Press: Ithaca, 1953.
[1.53] Silberberg, A. J. Chem. Phys. 1968, 48, 2835.
[1.54] Gregory, P. "High-Technology Applications of Organic Colorants"; Plenum
Press: New York, 1991.
CHAPTER 2
EXPERIMENTAL PROCEDURES
This chapter provides a description of the experimental processes and techniques
used in this study. Materials used to construct layer-by-layer deposited multilayer films
and the synthesis of Ru complex based materials will be explained. A description of film
characterization techniques is included. Finally, the fabrication and measurements of a light
emitting device (LED) will be explained.
1. Materials
1.1
A.
Polyelectrolyte Solution Preparation
Poly(aniline) Solution
Polyaniline (PAN) solutions (the PAN was synthesized using the procedures
reported by Chaing and MacDiarmid [2.1]) were prepared by dissolving PAN in dimethyl
acetamide (DMAc) at 20 mg/ml. The solution was then stirred overnight, followed by 8-10
hours of sonicating. The solution had some fine particulates which were removed by
filtering through a 2jim filter, followed by a 0.45gtm filter. The PAN dipping solution was
prepared by slowly adding one part (by volume) of the PAN solution in DMAc with 9 parts
water. The pH of the PAH solution was adjusted to about 3.0-3.5 with methane sulfonic
acid (MeSA).
The pH was then quickly lowered to 2.5-2.6 by adding drops of
concentrated MeSA.
B.
Poly(thiophene-3-acetic
acid) Solution
Poly(thiophene-3-acetic acid) (PTAA) was prepared by the ferric chloride
polymerization of ethyl thiophene-3-acetate (Lancaster Synthesis) followed by acid
hydrolysis of the ester group [2.2]. Dipping solutions with a PTAA concentration of
0.01M (based on unit molecular weight) and a pH level of 4.8 were used with 0.4 M of
NaC1.
C.
Other Polyelectrolyte Solution
Poly(vinylbenzyltrimethylammonium) (PVBT) was supplied by Prof. Otto Vogl
of Polytechnic University, Brooklyn, NY.
Poly(methacrylic acid) (sodium salt,
Mw=50,000) (PMA), Poly(acrylic acid) (Mw=90,000) (PAA), poly(diallyldimethyl
ammoniumchloride) (High Mw version) (PADA) and poly(styrene-4-sulfonate)
(Mw=75,000) (SPS) were obtained from Polysciences. Poly(allylamine) (Mw=28,000)
(PAH) was obtained from Aldrich. All polyelectrolytes were used as received, without
further purification. Those polyelectrolyte dipping solutions of 10-2 M were made in
Millipore water and pH adjusted with HCl and NaOH. Salts, such as MgCl 2 and NaC1,
were dissolved in the polyelectrolyte solutions to enhance polyelectrolyte adsorption.
1.2
Dye Solution Preparation
Congo red dye, Porphyrin dyes, Crystal Violet dye, Ponceau S, and Bordeaux R
dye were purchased from Aldrich. Infrared absorbing dye and Fluorescein complexion
were obtained from Kodak. All dyes were used without further purification. The dyes
were dissolved in Millipore water to give 10-2 M - 10-3 M solution. Those dyes have at
least two ionic groups in chemical structures. Again, pH was adjusted using HCl and
NaOH solutions.
1.3
Sulfonated tris(phenanthroline)ruthenium (II) dication complex,
([Ru(phen')3] +2 ) Synthesis [2.3]
[Ru(phen')3]+ 2 C12 was prepared by slight modification of a reported method
[2.4] . The reaction scheme of [Ru(phen') 3 ]+2C12 is shown in Figure 2.1. Commercial
RuC13.xH 2 0 was dried in a vacuum oven at 150'C for 3 hours. It was then ground finely
in a mortar and dried in the oven for further two hours prior to use. A mixture of predried
RuC13 (311 mg, 1.50 mmol) and anhydrous disodium salt of 4,7-diphenyl-1,10phenanthrolinedisulfonic acid (2.41 g, 4.50 mmol) in 60 ml of water was then refluxed for
60 minutes in the presence of a reducing agent (NaH2 PO 2 , 0.15 g, 1.66 mmol). The
resulting dark-orange solution was concentrated under vacuum and the product precipitated
into acetone. Yield was quantitative and the product was purified two or three more times
SO3Na
NaO 3S
RuC13
NaH 2PO 2
+
-N
N =-
2+
NaO 3
2C-
NaO 3
SO 3Na
(I)
Figure 2.1. Sulfonated Ru(phen')3+ 2 Cl 2 (I) synthesis scheme.
by recrystallization from methyl alcohol into acetone. The yield was more than 99% after
the product was dried in air.
1.4
Poly(tris(bipyridine)ruthenium(II)dication complex),
(poly([Ru(bpy)3]+ 2 )ester) Synthesis
A polyester type of Ru(bpy)3 + 2 was supplied from Jin-Kye Lee in our group.
The detailed synthesis of the polymer is described elsewhere [2.5]. However, the basic
routes of the synthesis of poly([Ru(bpy)3]+ 2 )ester include the step-polymerization of a
difunctionalized Ru(II) complex, Ru(bpy)2[bpy(CH20H)21+ 2 , with dodecanedioyl
dichloride, CIOC(CH 2 )10COC1. The final step of the synthesis scheme is shown in Figure
2.2.
§ OH
HO
~N
NI\
2+
N- Ru- N
N
(PF6 )2
+
CICO(CH 2) 10COC1
N
OO
CH 3CN/THF (1/1)
OCO(CH 2) 10 CO
N- Ru-N
N
X2
N
n
X = PF6-, soluble in acetone and acetonitrile
C1- , soluble in water and methanol
(II)
Figure 2.2. poly([Ru(bpy)31+ 2 )ester (II) synthesis scheme.
2.
Substrate Preparation
The clean surface was required for uniform adsorption. Microscope glass slides
treated to render their surface hydrophilic and hydrophobic were used for the dipping
procedure.
2.1
Hydrophilic Slide Substrate
Commercially pre-cleaned glass substrates were first placed in a concentrated
sulfuric acid (H2 SO4)/Hydrogen peroxide (H2 0 2 ) (7:3) bath for 1 hour and then in a
H2 0/H 2 0 2 /NH 3 (5:1:1) bath for 30 minutes. The substrates were extensively rinsed with
Millipore deionized water (resisitivity > 18Mg) after each cleaning step. The substrates
were dried with filtered compressed air and further dried at the 90TC for 1 hour. These
slides were stored in a plastic case prior to use.
2.2
Hydrophobic Slide Substrate
Hydrophobic surfaces were created by a gas phase treatment of hydrophilic
slides with 1,1,1,3,3,3 disilazane (HMDS) obtained from Aldrich. Its structure is shown
in Figure 2.3. The cleaned slides and 1 ml of HMDS were placed in a desiccator which
was then evacuated and maintained at a static vacuum (200 mTorr) for 36 hours. The
liquid disilazane was placed into a small evaporating dish and placed inside a vacuum
dessicator. Once the vacuum reached 200 mTorr, the chamber was sealed to keep a static
vacuum for 36 hours. After the disilazane treatment, the slides were rinsed with Millipore
water and hexane to remove residual HMDS. The contact angle of the hydrophobic
substrates was around 65 degrees.
2.3
Positively Charged Slide Substrate
Positively charged surfaces were fabricated by a solution treatment of the cleaned
hydrophilic slides with (N-2-aminoethyl-3-aminopropyl)trimethoxysilane (TMS) which
was supplied by Huls America, Inc. The cleaned slides were immersed for 10 minutes
each into the following solvents: methanol (HPLC grade, Mallinckrodt),
1:1
methanol/toluene, toluene (analytic reagent grade, Mallinckrodt). After rinsing with
methanol and toluene, the cleaned slides were exposed to a 5 volume% solution of TMS in
toluene for 15 hours. After silazation, the slides were immersed in fresh boiling toluene for
1 hour. Then they were immersed for 10 minutes each in toluene, 1:1 to toluene/methanol,
and methanol. Finally, they were extensively rinsed with Millipore water to remove
residual TMS and dried. This produced a surface with covalently anchored amine
functionalities; these functionalities were protonated in the low-pH solutions.
CH 3
CH 3
CH 3 -Si -- N--Si -CH
I
NHCH 2 CH 2NH 2
I
I
I
I
I
3
CH 3 H CH 3
(a) 1,1,1,3,3,3 hexamethyl disilazane
CH 2
I
CH 2CH 2Si(OMe) 3
(b) N-(2-aminoethyl-3-aminopropyl)
trimethoxysilane
Figure 2.3. Chemical structures of (a) hexamethyl disilazane (HMDS) and (b) N-(2aminoethyl-3-aminopropyl) trimethoxysilane (TMS).
3.
Layer-by-Layer Sequential Adsorption Technique
The layer-by-layer deposition technique used to create alternating multilayer thin
films involves repeated dipping of a substrate into dilute polycation and polyanion
solutions.
3.1
Hand Dipping Procedure
Films were deposited by alternately dipping substrates into polyelectrolyte
solutions. The substrates were dipped in the polyelectrolyte solution for 15 minutes and
subsequently rinsed with pH 2-4 solutions, which was adjusted to be the same pH of the
dipping solutions, until most non-adsorbed material was removed. After each deposition
and rinsing, the samples were blown dry with a flow of compressed, filtered air. The
substrate was then dipped in the oppositely charged solution, for an equal amount of time
followed by the rinsing and drying procedures. The deposition was performed at room
temperature.
3.2 Automatic Dipping Procedure
The layer-by-layer fabrication of multilayer thin films was carried out via the use
of an automatic dipping machine in the form of a programmable slide stainer (HMS
programmable slide stainer/Zeiss Inc.). This dipping machine was programmed to
immerse substrates (glass or patterned ITO) into a series of up to twenty different
solutions. The substrates were dipped into a polyelectrolyte solution followed by three
agitated rinses in three separate bins of pH adjusted water. The substrate was then dipped
into the oppositely charged polyelectrolyte solutions followed by these 3 separate rinses
with pH adjusted water. The dipping time in polyionic solutions was controlled for 15
minutes, and the rinsing times were 2 minutes for the first two rinses and 1 minute for the
third rinse. No drying step was used in the automatic dipping procedure.
4.
Characterization Methods
4.1
Contact Angle Measurement
An Advanced Surface Technology video contact angle measuring device was used
to measure the contact angle of water on deposited layer films. The contact angles
presented in this thesis are advancing contact angles which were measured with the sessile
drop technique [2.6 - 2.8]. An advancing contact angle (GA) is the kinetically stationary
angle formed on the surface after a water drop have advanced on the surface. A water drop
was made on the tip of a syringe and placed on a sample by moving the substrate vertically
until contact was made between the water drop and the sample. The subsequent addition of
a small amount of water to the water drop on the surface produced the static advancing
angle with the surface in a few seconds. The contact point of the syringe was kept at the
same position on the water drop. The needle was raised very slowly to prevent
deformation of the water drop. An image of the droplet was taken though the CCD camera
and enlarged. The right and left contact angles of the droplet were then measured using an
enlarged image transmitted to the computer screen. The size of water droplets was
maintained constant at 11l. (when the surface is homogenous, the size of the water drops
should not affect the value of the contact angles [2.6 - 2.8]) Before measurement of the
contact angle, the samples were dried at 90"C for 1 hour and stored in ambient air for 2
days. When the contact angles of consecutively deposited layers were measured, 12
separate samples were prepared where the total number of layers per sample varied from 1
to 12. Four separate locations (2 measurements each) on the sample were measured to
ensure a representative value of the contact angle. The average value of the measured
contact angles was used to represent the wetting characteristics of the sample.
4.2
Thickness Measurement
Multilayer films with 10, 20, 25 and 30 bilayers were built up on silicon wafers
and hydrophilic glass slide substrates. The films on the silicon wafers were used to
measure the total thickness of the films via ellipsometry because the smooth surface reduces
the noise in the light reflected from the substrate surface. Films on glass slides were
scratched for profilometry measurement. The depth of the scratch was then measured.
Thickness increment measurement was based on the consecutive layer depositions
onto the 10 and 11 layered samples. After each layer deposition, the thickness was
measured with 477 and 633 nm laser sources. Three additional layers were deposited onto
the 10 and 11 layered samples. The averaged values were then chosen to represent the
incremental thickness of the film.
A.
Profilometry
Profilometry is a mechanical contact method. The thickness of a film is measured
by scanning a scratched in depth profile with a diamond stylus. The films were scratched
with a razor blade to form approximately 1000
A lines.
By profiling the scored line, the
depth of the line was measured. The average value of five measurements was chosen to
represent the thickness of the film. A Sloan Dektak 8000 profilometer was used and the
stylus was set to the pressure of 10 mg to minimize error and noise. The Dektak
profilometer was used to measure films of 200 A and thicker.
B.
Ellipsometry
Ellipsometry is a sensitive and non-destructive method for measuring the
thickness of thin films. Ellipsometry can be used to give direct measurements of the
thickness and refractive index of thin films on various surfaces, including metals,
semiconductors, insulators, and polymers [2.9 - 2.10]. The technique is based on the
measurement of changes in the ellipticity of polarized light caused by reflection at an
interface. These changes are strongly influenced by the presence of a film on a smooth
reflecting surface. The ellipticity can be described by the ratio and phase difference of two
plane waves. The light wave can be viewed as a superposition of two plane waves, one of
which has the electric vector in the plane containing the wave vectors of the incident light
and the other with the electric vector at right angle to this plane. These two components
change as a function of the refractive index and the film thickness.
Ellipsometry is suitable for the measurement of films whose thickness is much
less than the wavelength of light. In a typical ellipsometer, monochromatic light is plane
polarized (P = angle of polarization) and reflected on the film surface. A reflected beam
enters the compensator and is elliptically polarized to plane-polarized (A = angle of
polarization). The angle A is determined by the analyzer. These two angles (P,A) give the
phase shift between the parallel and perpendicular components (A), and the changes in the
ratio of the amplitudes of the two components (tan y). The differential values of A, y are
related to the ratio of the reflection coefficients of parallel (rp) and polarized (r s ) light,
respectively,
n= tan
rs
-exp(iA)
(2.1)
As the films have different refractive indices from the substrate, the film thickness
is measured by calculating the difference between the clean substrate surface and the
substrate with films. In principle, for thickness > 50 A, ellipsometry can determine both
the thickness and reflective index. At least five points were measured and averaged for the
value of thickness. The data variance was within 10 A. At least two different wavelengths
of light were examined to measure the thickness of a sample.
4.3
Visible and Near ultraviolet Spectroscopy
Visible and near ultraviolet (UV/Vis) spectra were generated on an Oriel Multispec
spectrophotometer. Since PAN, PTAA and the dye molecules have distinctive colors,
UV/Vis spectroscopy is a powerful tool to check the amount of adsorption, proof of linear
adsorption, and chain orientation. Upon checking the UV/Vis spectral peaks of each layer,
the amount of adsorption was correlated with the number of functional groups on the
surface.
The chain orientation of molecules in films can be determined by using polarized
light spectroscopy. By comparing the absorption of light parallel (P) to the plane of
incidence with that of light perpendicular (S) to the plane of incidence, the average
orientation of a chromophore is determined [2,11,2.12]. The relations of the ratio of P
absorption (Ap) to S absorption (As) is given in the below equation (2.2).
AP
nICOSi+n3*COST
f1 3 n3
sin2i
.
osi + n os r (cos i cos r +
A=
As nl.cosr+n3 os i
n24 tan2
)
(2.2)
where n, is the refractive index of phase P (1=air, 2=polymer film, 3=glass substrate), i is
the angle of incidence at the air-film interface, and r is the angle of refraction at the filmglass interface. The dichro ratio is useful for making relative comparisons among different
conditions.
In order to check amount of carboxylic acid groups on a PAH/PAA multilayer film
structure, positively charged methylene blue dye (obtained from Aldrich) was applied to
dye multilayer films. The methylene blue did not deposit on bare slides. The amount of
the methylene blue adsorption was traced with the UV/Vis spectroscopy. The PAH / PAA
films with different number of layers were immersed in 10-3 M methylene blue solutions
for 10 minutes. After the dye immersion, the dyed multilayer films were soaked in water
(pH=4.0) for 1 minute and dried with a mild air flow.
4.4
Infrared Spectroscopy
Infrared spectroscopy was performed to study interactions in the polymer layers,
particularly where hydrogen bonding or acid-base interactions were concerned. The
vibrational energies of the polymer functional groups are changed by interactions between
layers. The resulting vibration energy difference appears as shifts in the vibration peaks of
the infrared spectra. If the bonds are stronger, vibrational peaks move to lower wave
number.
There is a strong tendency for hydrogen bonds to form if a hydrogen atom
attached to an adjacent atom, such as O or N, which carries a local concentration of
negative charge. The most important effect is a lowering of the stretching frequency of the
hydrogen bonded OH or NH group with respect to that of the 'free' groups [2.13].
The carboxylic peak in infrared spectroscopy can be used to check interactions
between the layers by relative amounts of carboxylate -COO-NH 3+ and hydrogen-bonded
-COOH-NH 2. Compositional heterogeneity was compared based on 1560 cm-1 of -COOand 1700 cm- 1 of -COOH [2.14, 2.15].
Similar to the UV/Vis spectroscopy, infrared spectroscopy is an appropriate
method to check the amount of adsorption, especially for materials that do not have peaks
in the UV/Vis range. A polished Zn/Se plate was used as a substrate for the layer-by-layer
deposition. Distinct functional group absorptions can also determine the amount and the
linearity of adsorption.
4.5
Fluorescence and Electroluminescence Spectroscopy
Photoluminescence and electroluminescence spectra of Ru complex incorporated
films were measured using a SPEX 312 scanning fluorimeter with CCD detector.
Photoluminescence spectra were taken by exciting the maximum absorption wavelength of
Ru complex materials. The excitation wavelength of Ru complex based samples was
chosen to be 470 nm. The samples were placed at the front face. The resulting
photoluminescence was collected by mirrors facing the substrate. The slits were typically
set to 1.0 mm for the source and the detector. The photoluminescent peak in Ru complex
materials was measured at 630 nm.
The electroluminescence of the Ru complex based light emitting devices (LEDs)
was measured in ambient air. The sample LEDs were positioned to face the detector. Then
the wire leads from a manual extended power source was connected. The devices were
turned on and the CCD carray collected the light for one second. The electroluminescence
spectrum was recorded with correction factors for the CCD detector.
electroluminescent peak of Ru based LEDs was determined to be 630 nm.
The
5. Light Emitting Device Fabrication
5.1 Indium tin oxide (ITO) Substrate
Electroluminescent devices were fabricated on glass substrates patterned with 2
mm wide, 2000 A thick lines of indium tin oxide (ITO) electrodes. The ITO substrates for
device fabrication consisted of two or four ITO lines. The ITO substrates were received
from TDK, Japan. The two point resistance of each ITO line is about 10 Ohms. These
substrates were scored with a diamond cutter and broken into pieces of appropriate size.
The ITO patterned substrates were sonicated in a 1:3 Lysol® (generic household cleaner):
Millipore water solution for 15 minutes. Then, the substrates were rinsed with fresh
Millipore water. The substrates were then placed in Millipore water and sonicated for 15
minutes. This pure water rinsing and sonication steps were repeated three more times. The
substrates were sonicated in acetone and ethanol for 15 minutes each. These cleaned
substrates were used immediately or stored under vacuum to maintain the clean surface.
5.2 Film Fabrication on ITO substrate
The cleaned ITO substrates were dried under vacuum at 60"C before use. The
spin-casting method or the layer-by-layer deposition technique were used to build up LEDs
in this study.
The spin-cast LEDs of Ru complex materials were fabricated by dissolving the Ru
complexes in 2-methoxy ethanol and then filtering through a teflon coated pm syringe
filter. The Ru complex solutions were dropped onto the cleaned ITO slide until the whole
surface was covered. The substrates were then spun at 3000 rpm for 1 minute, allowing
the solvent to evaporate. Any residual solvent was removed under vacuum.
The cleaned ITO substrates for the layer-by-layer deposition technique were pretreated with 1 layer of a polycation, (PAH or PEI), rinsed with Millipore water, and dried
with an air gun. Then a layer of polyanion was deposited onto the polycationic layer. The
negative or positive charges were established on the ITO surfaces. Automatic dipping or
hand dipping methods explained in previous sections were then used to form the multilayer
film.
5.3 Al Evaporation
2 mm wide lines of Al were deposited perpendicular to the ITO lines via thermal
evaporation at a pressure of 2.10-6 Torr. The size of LED cells was thereby determined to
be 4 mm 2 . The aluminum was evaporated at a rate of 250 A per second onto the films by
using 6 Al clips. The resultant thickness of the aluminum lines was typically between 2000
- 2500 A. Each sample therefore has two or four ITO lines and two or four Al lines. The
intersection made four or sixteen active devices that can be addressed individually by
contacting the appropriate gold leads to a power source. ITO sides were connected with the
positive charge and Al sides were connected with the negative charge of the power source.
The samples were placed under dynamic vacuum of 100 mTorr until they were ready to be
tested.
5.4
Current-Voltage and Light-Voltage Measurement
The samples were mounted onto L shaped glass slides. Gold wires were taped
down on the glass slides and were electrically contacted to the ITO or Al lines by silver
paste. Once the sample was tightly mounted onto the glass slides, the sample was placed in
a dark, controlled, nitrogen atmosphere. Inside the dark nitrogen atmosphere, light and
current were measured with a silicon photodiode (Model 1830-C from Newport
Instruments) and current meter (Hewlett Packard 34401A). The operation voltage was
applied to the devices using a programmable constant voltage source (Model 636 from
Keithley). The typical distance between an active cell and the detector was 1 cm. A 3 mm
by 3 mm mask was used to exclude unwanted light from the detector.
The current-voltage and light-voltage curves were taken automatically by a custom
LabView program written by our group. The LabView program controlled a series of
commands to ramp the voltage from zero to a designated maximum voltage. The step
voltage was 0.5 volt and delay per each voltage increment was 5 seconds. The delay per
each voltage increment gave some time to reach equilibrium values after a new voltage was
set. Once the voltage reached the designated maximum voltage, the program ramped down
the voltage with the same voltage step and the same delay per step. If the designated
voltage was a negative value, the program proceeded to ramp the voltage down from zero
to the maximum negative bias first and then increase the voltage from the maximum
negative bias to the zero with the same step. The current (mA) and the light intensity (nW)
at each voltage step were recorded automatically through the program and plotted with a
voltage axis. Each active light emitting cell was scanned multiple times until the light and
current versus voltage curves were identical in the upward and downward scans. The
hysteresis between the upward and downward scans is due to a charging effect in the Ru
complex based LEDs.
5.5
Device Lifetime Testing
The life-time and stability of the device were investigated at the same voltage.
The LED devices were again operated under the nitrogen gas and maintained at the
designated voltage. Each measurement of current and light values were read according to a
user defined "sampling time interval".
5.6
Efficiency of Device Performance
One of the most important parameters for evaluating LED behavior is the EL
efficiency. The efficiency of the devices is equal to the average number of photons
produced for each electron. The external and detectable light output was used to calculate
an external quantum efficiency. The losses and reflected light output at the ITO and glass
substrates were not observed. An internal quantum efficiency only can be calculated by
using "correction factors".
The flux of photons was calculated by dividing the light power output reading by
the energy per photon. The energy per photon was averaged based on the center
wavelength of the electroluminescence peak of Ru complex materials (630 nm). The
number of electrons per second can also be calculated by dividing the current by unit charge
current. The ratio of photon flux and electron flux gives the external quantum efficiency.
However, the small size of the photodiode used may not detect all the light output from
LED cell. A correction factor to convert the collected light output to the real light output
based on the distance and the angle between the detector and a LED cell. This was
calculated by the similar method in the literature [2.16]. The calculated correction factor
shows that the detector only collects about 27% of the light emitted. The refractive index of
the ITO substrate was assumed to be 1.5.
Pc(corrected light output) = Pm(measured light output) / 0.27 (nW)
Photon Flux = (Pc / hv) (Photon / second)
(2.3)
(2.4)
where h is the Frank's constant and v is the wavelength of light output.
Electron Flux = (I / e) (electron / second)
(2.5)
In this equation, I is the measured current (mA) and e is the charge current. The external
quantum efficiency (EQE) is calculated as the values of photon flux and electron flux.
EQE(%) = [(I / e) / (P
/
hv)] * 100
(2.6)
Maximum light output was a practical important parameter. The light output in
nW is converted into cd/m 2 through the optometer measurement (S370 from Graseby
Optronics). The approximate conversion factor for the Ru complex based devices were
determined as 25 - 30 nw=1 cd/m 2 .
REFERENCES
[2.1]
Chiang, J. H.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193.
[2.2]
Royappa, A. J.; Rubner, M. F. Langmuir 1992, 8, 3168.
[2.3]
Lee, J-K.; Yoo, D.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett. 1996, 69,
1686.
[2.4/
Inorganic Synthesis 1990, 28, 338.
[2.5]
Lee, J-K.; Yoo, D.; Rubner, M. F. Submitted in Chem. Mater. 1997.
[2.6]
Zisman, W. A. in "Contact Angle, Wettability, and Adhesion"; Advance in
Chemistry Series No. 43; American Chemical society: Washington D.C., 1964, pl.
[2.7]
Schrader, E.; Loeb, G. I. "Modem Apporach to Wettability"; Plenum Press: New
York, 1992.
[2.8]
Mittal, K. L. "Contact Angle, Wettability and Adhesion"; VSP: Utrecht, 1993.
[2.9]
Azzam, R. M. A.; Bashara, N. M. "Ellipsometry and polarized light"; North-
Holland Publishing Company: Amsterdam, 1977.
[2.10] Ulman, A. "An Introduction to Ultrathin Organic Films from Langmuir-Blodgett
to Self-Assembly"; Academic Press: San Diego, 1991.
[2.11] Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378.
[2.12] Vandevyver, M.; Barraud, A.; Teixier, R.; Maillard, P.; Gianotti, C. J. Colloid
Interface Sci. 1982, 85, 571.
[2.13] Eklind, H.; Hjertberg, T. Macromolecules 1993, 26, 5844.
[2.14] Bower, D. I.; Maddams, W. F. "The Vibrational Spectroscopy of Polymers";
University of Cambridge Press: Cambridge, 1989.
[2.15] Coleman, M. M.; Su, Y.; Painter, P. C. Macromolecules 1994, 27, 127.
[2.16] Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. Adv. Mater. 1994, 6, 491.
CHAPTER 3
SURFACE MODIFICATIONS AND LAYER STRUCTURES OF
LAYER-BY-LAYER SEQUENTIAL ADSORBED MULTILAYERS
1. Introduction
Surface properties of materials play important roles in applications such as paints and
coatings, adhesives, lubrication, biocompatible materials and separation methods [3.1 - 3.5]. In
these applications, high-energy solid surfaces promote the adhesion of solutes such as polymers,
inks, paints, proteins, and cells, while low-energy solid surfaces provide chemically inert and nonwettable interfaces.
Several methods have been developed to modify the surfaces of polymers and siliceous
surfaces [3.1]. The incorporation of surfactant onto the materials, corona discharge [3.6, 3.7] and
other low-pressure plasma treatments [3.8 - 3.11], as well as chemical methods [3.12 - 3.15] have
all been used to incorporate functional groups, capable of interaction with liquids, onto the surface.
The disadvantage of using common surfactants is that the hydrophilic properties rendered
by such materials are easily destroyed by the surfactant being dissolved in water. The corona
discharge and cold plasma treatments are currently being used in industry to implant oxidized
functional groups thereby creating high energy surfaces which work to enhance coating, printing
and adhesive bonding. The drawbacks of these techniques are the severe conditions required for
the impingement of various ions, radicals, and electrons onto the surface. This treatment also does
not produce well-defined surfaces due to various reactions at the surface. In addition, the prepared
surfaces are relatively unstable due to the high concentration of functional groups at the solid-liquid
or solid-gas interface. This high concentration causes the functional groups to differ away from
the interface. Therefore, the modified substrates should be used in a short time (about a few days)
to maintain the modified surface properties.
Chemical methods are also possible ways to modify the wettability of a solid surface, but
the problems associated with this technique lie in the severe reaction processes required to generate
functional groups on the surfaces. The technique seems to be impractical for a wide range of
applications. These severe chemical reactions influence not only the surface but also the bulk
properties of polymeric materials. A simple technique for surface modification that can be
maintained for an extended time is therefore an important topic for many researchers.
A novel layer-by-layer sequential adsorption technique has been introduced to modify
surface properties. This technique makes it possible to apply to any surface (e.g., hydrophobic,
hydrophilic, films with various surface groups, etc.) a thin polymer film in a layer-by-layer manner
using aqueous solutions. This sequential adsorption technique was considered as a possible way
to modify surface properties because of the well established fact that the surfaces of many different
types of materials can be dramatically modified by the adsorption of a thin film of a polymeric
material [3.16, 3.17]. In this new deposition mode, layers of polycations and polyanions are
sequentially adsorbed onto a substrate surface by alternate dipping the substrate into dilute aqueous
solutions of these polyelectrolytes. The ionic attractions developed between the oppositely charged
polyelectrolytes promote a strong interlayer adhesion and a uniform and linear deposition process.
In this dipping procedure, individual layers have their own functional groups which can be used to
achieve desirable surface properties. Polyelectrolytes with strong adsorption to a surface are
suitable for the first layer, while control over the desired surface property is obtained by the
consecutive layers.
The first part of this chapter will show that the wettability of a surface can be
systematically controlled via the deposition of suitable multilayers of polycations and polyanions.
For example, the fabrication of a completely wettable surface on siliceous substrates through the
use of this layer-by-layer deposition technique will be discussed. If the surface is hydrophilic
enough to inhibit the creation of water droplets on the surface, anti-fogging and easy cleaning
behaviors can be obtained through the strong interactions between water and the modified surface.
Polyelectrolytes are typically adsorbed onto a surface from a dilute aqueous solution with an
adsorption time of about 15 minutes usually being sufficient to achieve the complete coverage. The
amount and structure of the deposited polyelectrolyte in each layer is determined by the nature of
the polyelectrolyte/surface interaction and the condition of the dipping solutions. We can control
the adsorption to form either a more extended chain and consequently thinner layer, or to favor a
more coiled chain and thicker layer.
In the second part of this chapter, the systematic changes of the layer thickness and the
level of interdiffusion of layers in the layer-by-layer deposition of multilayer films will be
examined. Various films of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA)
were produced by controlling pH, ionic strength and concentration of the polyelectrolyte solutions.
PAH and PAA layers were introduced because these two layers have distinct wettabilities and are
sensitive to the dipping condition. Incremental thickness measurements and the selective
adsorption of cationic dye molecules onto the multilayer films were need to probe the adsorption
behavior of the PAH and PAA. The use of static water contact angles demonstrated that the
surface wettability showed a layer-by-layer oscillation and the amplitude of these oscillations in the
even-odd number layer pattern reflected the different levels of interdiffusion of the two layers.
2. Contact Angle and Wettability
2.1
Definitions
Contact angle measurement is one of the most convenient and most sensitive methods of
characterizing a solid/liquid interface as it directly measures the fundamental energies at an
interface, although it is not easy to get precise data or to interpret the experimental data. The
relationship between surface tension and wetting was first discussed by Young [3.18] in 1805. A
small drop of a liquid was placed onto a flat level surface of a film and an angle between the drop
i
and the surface at the air-liquid-surface interface was measured (depicted in Figure 3.1). The
contact angle is considered to be the result of the mechanical equilibrium of the drop resting on a
plain solid surface under the action of three surface tensions- y1, at the interface of the liquid and
vapor phases, y,, at the interface of the solid and liquid phases, and y,, at the interface of solid
and vapor phases. Hence, for a liquid drop on a solid surface, the equilibrium situation is given by
Young's equation (3.1).
COSe
From Young's equation, COS
= yS
- Ys1
"vV
(3.1)
is proportional to the difference between 'y, and 'st. The value
of y,1 does not depend on the surface. This means that the interaction between a surface and a
liquid drop dominates the value of the contact angle. The lowest energy state will correspond to
the liquid spreading out to cover as much of the solid surface as possible.
Figure 3.1. A sessile liquid drop on a flat surface indicating contact angle.
The direct measurements of the contact angle can be used to give a definition to the notion
of wettability and indicate the surface parameters or interfacial tensions such as ylv, Ysl, and y,,
which cannot be directly measured.
Suppose ', - y•s, 5 7v, then Young's equation may also be written
COSO = 7s- 7es-
71 - 7elv
s
(3.2)
where the nr terms are defined by the following equations.
s - 7, + is,,
7Y- 7v +7tev
(3.3)
(3.4)
Here, y, and y, are the surface free energies of the separate phases in contact only with their own
vapor, and the re terms are the equilibrium spreading pressures. The film pressure at the liquidvapor interface, 7rit
,
is commonly ignored because the adsorption at the liquid-vapor interface will
be small if the solubility of the solid in the liquid is very low. The rs,, term is measured
experimentally, and its value can be negligible for molecularly smooth low-energy solids, when a
liquid forms non-zero contact angle. The measurement of 7re is not a simple matter on a
macroscopic solid surface. The two 7re values are tacitly assumed to be negligible [3.19]. In the
approximation that tre, and re/, can be neglected, Young's equation becomes a simplified equation
(3.5).
COSO =_ 7'S
7, - 7s' 2 71
71
- 7 5
(3.5)
if COS= 1
(3.6)
Young's equation is now a function of experimental data, such as the surface tension of a
liquid drop and a solid surface. In principle, we can extrapolate the data of 7s, based on measured
contact angles by using the known value of 7y, and measurable value of 7,. In reality, however, it
is still hard to differentiate between 7, and 7,s in the contact angle data because the heterogeneity,
reorientation, solubility and interaction of the surface all affect the wetting behavior of a liquid
drop.
To access the wetting behavior in terms of the surface tension of the liquid, the equation
based on adhesion forces was suggested. The equilibrium energy of adhesion is the reversible
force required to separate a unit area of liquid from a solid. It is defined by the Young-Dupr6
equation [3.20]
AG = -y (1+ cos 6)
(3.7)
Israelachivili and Gee suggested a more general experimental approach to the treatment of the
wetting behavior based on this equation [3.21]. AG, may be divided into two different forces;
Lifshitz-Van der Waals attraction (LW) and acid-base interaction (AB) [3.22]. Each force is
defined as follows.
a
AG" =AG"LW +AG~s
(3.8)
AGLW =-2/ ys•W yW
(3.9)
AG AB =-2
2
(3.10)
7s --2 y-•+
which yield the following important equation
y(l + cos)= 2 /ySW -y• + 2ý
+22-,
(3.11)
If we measure yLW, y- and y+, the contact angle is then defined. We can possibly map the
values of surface tensions through the several measurement of contact angles [3.23]. The values
of yLW terms do not vary to a material, and is smaller than the ionic interaction parameters, y- and
y'. The amount of acid-base interaction at the interface contributes the wetting behavior more
strongly.
In order to obtain simple quantitative correlation between the acid-base contribution in
equation (3.11) and the work of adhesion can be separated into the surface population of acidic or
basic sites, Nab, and the enthalpy of the acid-base interaction, AHab according to the Fowkes and
Mostafa equation [3.24],
Wab =
fNab . AHab
(3.12)
Here, f is a factor for converting the enthalpy to free energy. f is less than unity. The
number of interaction sites between the solid and liquid is important in determining the wetting of a
surface.
In the layer-by-layer deposition, there are many charged ionic groups on the surface of the
multilayer films. Some of these groups are directly connected to the underlying oppositely charged
layer and others are dangling and not attached to the surface. For dangling functional groups such
as, sulfonic acid and amine, as the drop advances over the surface, y,, would be compensated by
the relative amounts of ionized and deionized groups on the surface [3.25 - 3.27]. The work of
adhesion is equivalent to the negative free energy change associated with the contacting event or
adhesion of two phases, that is Wab = -AGa
The free energy of spreading is strongly affected
by the number of ionic groups on the surface. The extent of water wettability should depend
directly on the number of ionic interaction sites at the contact area. Surfaces which have an ionic
character are likely to be hydrophilic whereas surfaces of materials where covalent bonds or Van
der Waals forces predominate are likely to be hydrophobic. Only weak van der Waals forces exist
at the fluorocarbon surface which are the most hydrophobic of all common organic substances.
The latter part of the equation (3.11) goes to zero when unpolar polymers, such as polyethylene
and poly(tetraflouride), interact with liquids.
Water is considered to have the same tendency for deprotonation and protonation. y' and
y- of water are defined as value of 25.5. A high concentration of charge on the surface can
enhance its hydrophilic nature. In other words, the hydrophilicity of a surface is proportional to
the number of charges on the surface. The hydrogen bonding and acid-base interactions at the
interface are very important forces which can enhance wettability. This equation indicates that our
polyelectrolyte depositions can modify the wettability through the ionic interactions as well as the
functional group at the surfaces.
2.2
Sensitivity of Contact Angle Measurement
As "interphase" is defined to be the depth sensitivity by an analytical tool. The interphase
of wetting is in 5 - 10 A of a solid from the surface. The functional groups outside this region
from the surface do not influence the wetting behavior of the solid surface. In case that liquid can
penetrate into solid materials or dissolve surface, the interphase is deeper than 5 - 10
A region.
Contact angle measurement is one of the most sensitive methods of analyzing surface properties.
An advancing angle is measured by slowly adding liquid from a syringe to a drop already
on the surface. The advancing contact angle (OA) is the kinetically stationary angle formed after a
drop has advanced across a surface. The accuracy of the advancing angle measurement is
enhanced by the cautious addition of water onto the water drop. Agreement between different
individuals measuring the same drop will be about 1 -2*. It is recommended that measurements of
contact angle are often made on both sides of the several drops and the numbers averaged. A
receding contact angle (OR) is the kinetically stationary angle formed after a drop has receded
across a surface.
2.3
Surface Roughness Effect on Contact Angles
Solid surfaces are known to be inhomogenous on a molecular scale, as well as on a (sub)
microscopic or even macroscopic scale, both physically and chemically. Physical inhomogeniety,
i.e., surface roughness changes the area of the solid-liquid interface. One can define a true contact
angle (0true), an observed contact angle (0), and a roughness factor (r), which is defined as the
ratio between the true surface area and its geometrical area (r 2 1) and can be expressed by the
following relationship [3.28, 3.29]
COSO = r -COStrue
(3.13)
According to this equation, a rough surface will increase a contact angle for those that are greater
than 90* and decrease those that are less than 90".
A hysteresis, defined as the difference between the advancing and receding contact angle,
is strongly related to surface roughness. A measure of surface roughness can be given by the
difference between the contact angle, OA, observed in advancing the liquid boundary over a dry,
clean surface and the angle OR, observed in receding the liquid boundary over the previously
wetted surface. A rougher, nonpolar surface will increase the advancing contact angles and
decrease the receding contact angles and as a result, the hysteresis will also increase. Although the
roughening of the surface does increase the advancing angles, it is found that the effect of
roughness is much more pronounced in the measurement of receding angles. The results by [3.30,
3.31], suggest that the absolute value of advancing contact angles are more accurate in determining
the chemical composition of a surface [3.19]. Micron scale roughness can exhibit the observed
differences in contact angles, but the roughness of our multilayer film is under 100 A. It can be
assumed that the different contact angles observed in layer-by-layer deposited films come primarily
from the chemical composition on the surface. The advancing contact angles are quite reproducible
and accurately measure the surface properties and the relative amounts of functional groups
present. The contact angles can be interpreted in terms of the structure and composition of the
outermost layers.
3.
Surface Modification by Layer-by-Layer Sequential Adsorption
Technique
3.1
Surface Properties of 1 Bilayer of Several Polyelectrolytes
Sequentially adsorbed films with various polycations and polyanions were fabricated on
hydrophilic glass microscope slides. The adsorption process was begun by first depositing a
polycationic layer, such as poly(aniline) (PAN) or poly(allylamine hydrochloride) (PAH), onto the
slides to generate positive charges. Polyanions were then applied as an outermost layer to modify
wmCH 2 -CH~
GH2
wCH 2 -CHOO
I
NH 3
+
Poly(allylaminehydrochrolide) (PAH)
-
Poly(acrylic acid) (PAA)
iCH
3
H
IACH2 -C
--aI
COO
Poly(aniline) (PAN)
~-CH2 -CH 2 -NH
Poly(methacrylic acid) (PMA)
2
Poly(ethyleneimine) (PEI)
CH 2COO
Poly(thiophene-3-acetic acid) (PTAA)
~wCH 2 -CHv
vv-i
CH,
SO
3
Poly(diallyldimethyl)amine (PADA)
O
O
"O-C
M N-C
Poly(sulfonated styrene) (SPS)
C-O'
0-
C-
(n
on e
PMDA-ODA (unconverted)
PMDA-ODA (converted)
Figure 3.2. Chemical structures of polyelectrolytes used to construct multilayer thin films.
surface properties. The structures of the polycations and polyanions used in this study are shown
in Figure 3.2. We tried to determine the effect of various polyanions as the outermost layer on the
wettability of the surface. To answer this question, a series of different combinations of PAH or
PAN with other polymers was used for the surface modification. The surface hydrophilicity of a
single bilayer was determined by measuring the advancing contact angle of water drops on the
surface. The contact angle of uncoated hydrophilic glass slides was determined to be about 100 +
2.
Table 3.1 shows that single bilayer films made from combinations of polycations and
various polyanions produced different surface properties ranging from hydrophilic to hydrophobic.
The contact angles in table 3.1 indicate that by simply changing the outermost layer, significantly
different surfaces in terms of wettability were produced according to the chemical structure of the
polyions. In addition, the surface properties of the outermost polyions are also influenced by the
dipping conditions. As a result, the hydrophilic nature of the outermost layer determined the
wettability of the multilayer films. The carboxylic acid groups of poly(acrylic acid) (PAA) produce
completely wettable surfaces, while the aromatic groups of converted (PMDA-ODA),
poly(thiophene-3-acetic acid) (PTAA) and sulfonated poly(styrene) (SPS) produce hydrophobic
surfaces despite the fact that they have ionic groups in the side chains. Our group has recently
shown that the contact angle of the outermost layer of poly(phenylenevinylene) (PPV) was more
than 100' indicating that very hydrophobic surfaces are possible with this technique. The results in
Table 3.1 suggest that this layer-by-layer deposition is a unique way to control wettability onto a
surface. The different conditions of dipping solutions also changed the wettability of the 1 bilayer
samples.
As will be discussed shortly, the number of unbound charged groups, and the level of
interpenentration of each layer as well as the chemical structure of the polyions play important roles
in generating the different surface wettabilities. The fabrication of various PAH/PAA structures by
changing the conditions of PAH and PAA dipping solutions and a proposed model of the layer-bylayer deposition will be discussed in the latter part of this chapter.
Table 3.1. Different surface properties by changing polyanionic layers.
Hydrophilic substrates were used. No ionic strength adjusted in polyanionic solutions.
Contact Angle (degrees)
Combination of Bilayer Films
A. PAH as cationic layer
PAH / Poly(acrylic acid) pH 2.5 - Auto-dipping
1.
PAH / Poly(acrylic acid) pH 2.5 - Hand-dipping
2.
3.
PAH / Poly(methacrylic acid) pH 2.5 - Auto-dipping
4.
5.
6.
7.
8.
<5
18 ± 5
11 ± 2
PAH / Poly(methacrylic acid) pH 2.5 - Hand-dipping
PAH / Poly(3-thiophene acetic acid) pH 4.3
20 ± 4
PAH / Sulfonate Poly(styrene) pH 2.5
PAH / PMDA-ODA (unconverted) pH 3.8
PAH / PMDA-ODA (converted) pH 3.8
55 ± 2
37 ± 6
50 ± 3
72 + 3
B: PAN as cationic layer
1.
PAN / Poly(methacrylic acid) pH 2.5 - Hand-dipping
23 ± 3
2.
PAN / Poly(vinyl pyrrolidone) pH 5.3
34 ± 3
3.
PAN / Poly(vinyl alcohol) pH 5.3
35 ± 3
4.
5.
PAN / Poly(3-thiophene acetic acid) pH 4.3
PAN / Poly(sulfonate styrene) pH 2.5
51 ± 2
51 ± 2
* Polyaniline(PAN) Solution
Conc.
10-3 M, pH 2.5
* Polyallylamine(PAH) Solution
Conc.
10-2 M, pH 2.5
* Polyanionic Solution
Conc. 10-2 M
It is also observed that different methods of dipping generate different hydrophilic
surfaces of PAH/PMA and PAH/PAA. The automatic dipping method produced a completely
wettable surface (contact angle <50), while the hand-dipping method only produced a surface with
a contact angle of about 180. The automatic dipping process increased the concentration of PAA
and thereby the density of carboxylic acid groups on the surface. The completely wettable surface
successfully inhibited the formation of fog by blocking water aggregation on the surface. The
PMA layer showed a slightly higher contact angle as compared to the PAA layer. These
experimental results show that only 1 bilayer of adsorbed material can effectively modify the
surface properties and can be systematically controlled with a combination of polyelectrolytes.
In order to explore the effects of multiple layers and the subsequent layer dependence,
samples having from 1 to 12 layers of PAH/PTAA and PAHIPAA were built up and the wettability
of the samples measured. The contact angle data of these consecutive layers can give us some
information about the contribution of each layer to the wettability of the surface.
3.2
Surface Properties of PAH/PAA and PAH/PTAA Multilayer Films
The sequentially adsorbed layers of PAH/PAA on hydrophobic slide substrates and
PAH/PTAA on hydrophilic slide substrates were fabricated with a PAH solution (pH 2.5 , 10-2
M), a PAA solution (pH 2.5, 10-2 M) and a PTAA solution (pH 4.3, 10- 3 M, NaCl 0.4 M) using
both the automatic dipping and the hand dipping method.
Figure 3.3 shows that these deposited multilayers can be used to alter the surface
wettability of both hydrophilic and hydrophobic substrates. An odd number of layers indicates a
PAH as the outermost layer, whereas an even number of layers indicates PAA or PTAA as the
outermost layer. It can be seen in the PAH/PAA adsorbed layers that the contact angle
systematically alternates between a completely wettable surface to one with a contact angle around
280, depending upon the outermost layer (the polycation or the polyanion). The oscillation
continues to about the same degree even after 12 layers have been deposited onto the surface.
These results clearly show that each individual layer distinctly affects the wettability of the surface.
After only one bilayer of polyelectrolyte deposition, the surface of the substrate was modified to be
completely wettable. This oscillation in contact angle, however, is seen to be much less for the
case of the PAH/PTAA system as opposed to the PAH/PAA system. The small oscillation
observed suggest that the surface properties do not depend much on the outermost layer in the
PAH/PTAA system. It is believed that the reasons for the small contact angle oscillation are that
the two layers are heavily interdiffused and also have similar wettabilities. The thickness per
bilayer of the PAH/PTAA system was about 22 A and that of PAH/PAA (automatic dipping
method) was about 42A.
70
60
50
a
I-
40
30
0
20
r•
10
0
1
2
3
4
5
6
7
8
9
1011
12
Number of Layers
Figure 3.3. Contact angles of PAH/PTAA layers on hydrophilic slide glasses
and PAH/PAA layers on hydrophobic slide glasses.
For the hydrophilic substrate, it only takes 1 bilayer of the PAH/PTAA system to change
the contact angle from 10 to about 50, thereby rendering the surface less wettable. Similarly, a
hydrophobic surface can be modified to be a completely wettable by depositing 1 bilayer of the
PAH/PAA (10-2 M, pH 2.5 / 10- 2 M, pH 2.5) system through the automatic dipping method.
Figure 3.4. Contact angles of (a) hydrophobic slide glass and (b) 1 bilayer of PAH/PAA
on hydrophobic slide glass with PAA as outermost layer.
It is also observed that PAH layers in the PAH/PTAA system have higher contact angles
than the PAH layer in PAH/PAA system. The different wettabilities of the PTAA layer and the
PAA layer apparently affected the contact angles of PAH layers in the PAH/PAA and PAH/PTAA
multilayer films. The influence of the underneath layer on the wettability of outermost layers
suggests that layers are interdiffused up to some level, though the layer-by-layer wettability
dependence is still observed. The contact angles of PAH/PAA samples have shown the strong
dependence on the processing method. The automatic dipping methods did not apply the blow
drying station between dippings, instead, three water rinsings were applied. The thickness data
show that the thickness per bilayer of hand-dipped samples was about 33
A, while the auto-dipped
samples was about 42 A. It might be that the auto-dipping processing helped to keep the swelling
of polyions to enhance the amount of adsorption and soaking in water bin for 10 minutes which, in
turn, helped the ionized PAA layers to increase the number of sites, even though the number of
charges on the surface were titrated again with the pH of PAH solution when the samples were
dipped.
Figure 3.4 shows the pictures of water droplets on one bilayer of PAH and PAA and on a
hydrophobic slide glass. Only 1 bilayer, which had about 40 A thickness, successfully produced a
completely wettable surface. As long as the bottom layer generates sufficient positive charges on a
surface, the completely wettable surface will be obtained by the technique.
3.3
Acid-Base Interaction in Wetting
The completely wettable surfaces were produced by the carboxylic acid groups on the
surface. Those groups in polyions can be interconverted from CO 2H to CO 2- by changing the pH
of the solutions. If there is an acid-base reaction on the surface, the additional work goes to
increase the wettability of the surface. Whitesides et al intensively studied the interphase of selfassembled monolayer (SAM) structures of their films with titration contact angles [3.32 - 3.34].
By placing water drops of different pH on the surface, we can measure the pKa value of the
carboxylic acid groups on the sample with the PAA as the outermost layer. Because the
transformation from neutral to charged groups is more difficult in the interphase region, higher pKa
values in the solid state than those in the solution state were generally observed using this contact
angle titration method. If the pH of the water drops is higher than the pKa, we expect the acid-base
interaction at the surface. One of the interesting results of the study is that the higher concentration
of carboxylic acid groups on the surface decreased the actual pKa value at the surface. This implies
35
30
I-
25
0
20
4r
15
re
10
5
2
4
6
8
pH of Water Drops
10
Figure 3.5. Contact angle titrations of 3 bilayers of PAH/PAA with PAA as
outermost layers
that the transition of titration of contact angles was decreased with increase of the amount of
carboxylic groups on the surface. Even though the carboxylate form (COO-) induced more
hydrophilic character, a high density of unconverted -COOH groups also can work to make the
surface hydrophilic sites [3.35]. The lower pH (strong acid) cannot convert the number of the
carboxylic groups and therefore interacts less with water drops on the surface. Using contact angle
titration, contact angles of 3 bilayers of PAH and PAA, fabricated by both hand-dipping and autodipping, were measured with drops ranging in pH from 1 to 11. Figure 3.5 shows that acidic
water (pH < 3) produced higher contact angles than neutral and basic water. When the contact
angle was lower than 100, there was almost no dependence on the pH of the drops except for the
pH 1 drop [3.33]. Our results are comparable to the results of Whitesides et al where the surfaces
are perfectly covered by SAM structure with the carboxylic groups. From these results, it can be
concluded that the completely wettable surfaces were obtained primarily by the high density of
carboxylic groups on the surface and not from the work of acid-base conversion.
3.4
Fabrication of Completely Wettable Surface
Several different cationic polyelectrolytes, such as PEI, PADA and PAH were used for
building up multilayer films. If the polycation is deposited strongly on the surface, the next layer
is attached to the positive charges of the layer. As long as the first polycationic layer deposits a
sufficient number of charged sites on the surface, the outermost layer will deposit the same amount
and thereby change the wettability of the surface. Completely wettable surfaces were obtained
independent of the polycations. PEI and PADA both provided a sufficient number of charged
groups on the surface.
One and three bilayers of PAH/PAA were built up on hydrophilic, hydrophobic and
treated substrates. Because the treated substrate is positively charged, the PAA was the first layer
on the substrates. The 1 bilayer and 3 bilayer films of the three different substrates exhibited a
completely wettable surface regardless of the nature of the substrates.
The stabilities of the completely wettable surfaces were monitored by placing the
PAH/PAA samples in the air and in a 90TC oven. Figure 3.6 shows the aging of the contact angles
of 3 bilayers of PAH/PAA on hydrophilic and hydrophobic substrates with time. The PAA layers
retained their wettability at room temperature for more than 1 month but started to lose some of
their hydrophilic character after 2 months. Figure 3.7 shows that the contact angle of the
40
35
O 3 bilayers on Hydrophobic slide
* 3 bilayers on Hydrophilic slide
30
25
20
15
10
5
"4 0
U0
10
20
30
40
50
Number of days in ambient air
60
Figure 3.6. Aging effect on contact angles of PAH/PAA layers in the air.
An
7,
,
35
30
O
*
3 bilayers on Hydrophobic slide
3 bilayers on Hydrophilic slide
W
25
0
20
15
10
5
IC
0
5
10
15
20
25
30
Number of days in 90 "C
Figure 3.7. Aging effect on contact angles of PAH/PAA layers at 90 "C.
completely wettable surface, exposed to a 90'C static oven, retained its hydrophilicity for more
than 1 week. Compared with modified surfaces using different conventional techniques, the
layer-by-layer deposited films show excellent stability. The plasma treated polymer surface does
not last more than a few days due to the reorientation of the polymer chains at the interface [3.36].
The PAH/PAA films with PAA as the outermost layer have a high density of carboxylic
groups at the surface. The reorientation of the polymer chains at the surface, therefore, may not
influence the wettability of PAH/PAA layers as much as in the plasma treatment. The adsorption
of particles, such as dust as well as oxygen and the interdiffusion of the carboxylic acid groups
decrease the hydrophilic nature of the PAH/PAA surfaces. The completely wettable surfaces are
high energy surfaces that adsorbs contaminants from the air.
From previous work in our group and others, it is known that the layer thickness of the
adsorbed polyelectrolyte can be manipulated by the ionic strength and concentration of the dipping
solutions. The structure of the layers and the level of interdiffusion were studied by analyzing the
surface properties of the multilayer films. Based on the theory and experimental results of a single
layer polyelectrolyte adsorption on a charged surface, the adsorption behavior in multilayer film
buildup was tried to be understood. For this study, several sets of PAH/PAA layers were made
with different sets of solution conditions, which produced various thicknesses per bilayer and
degrees of oscillation of the contact angles.
4.
PAH / PAA Multilayer Film Fabrication
The automatic dipping method was used to build up a number of PAH/PAA layers on
hydrophilic slides. The linear growth of these films with various dipping conditions was measured
primarily by profilometry and confirmed through ellipsometry measurements of the films on a
silicon wafer. During adsorption, the charges of the deposited polyelectrolytes limit the adsorption
of additional polyions through electro-static repulsion at the interface. Figure 3.8 displays the
linear growth of PAH/PAA multilayer films up to 30 bilayers at various pH, ionic strength, and
concentrations of PAH and PAA solutions.
To understand the contribution of each layer to the thickness per bilayer of the PAH/PAA
samples, the thickness increment after each layer was measured on a silicon wafer by ellipsometry.
The thickness per bilayer and the incremental thicknesses are summarized in Table 3.2. The results
show that various dipping conditions produced different ratios of the PAH and PAA layers. When
the polyelectrolyte is likely to deposit as a loopy coil, a relatively thick layer will be developed. On
the other hand, when the polyelectrolyte is deposited as an extended chain, a thin layer will be
obtained. PAH, with pH 2.5 and 10-2 M, and PAA, with pH 2.5 and 10-2 M, produced a linear
thickness of about 42 A where the layer of PAA was about 29 A and the PAH layer was only about
13 A layer. The addition of a divalent salt, MgCl2, at a concentration of 0.4 M in the PAH solution
produced about 58 A per bilayer by increasing the thickness contribution of the PAH layers.
20VV
2000
o4
1500
t 1000
500
n3
0
5
10 15 20 25 30
Number of Bilayers
35
Figure 3.8. Linear growth of several sequentially adsorbed PAH/PAA films. Note
that the first parenthesis shows the condition (concentration, pH) of PAH solution and
the second parenthesis shows the condition of PAA solution.
PAH, with pH 4.5 and 10-2 M, and PAA, with a pH 4.5 and 10-2 M, produced even
thicker layers (about 65
A) with 47 A contributed
by the PAH and 18
A contributed by the PAA.
Compared to the PAA solution with a pH 2.5, the pH 4.5 of PAA solution produced a thin layer
with a more extended chain conformation. The thickest layer in the PAH/PAA systems was 72
A
per bilayer and was obtained with the combination of PAH at a pH of 4.5 and 10-2 M and PAA at a
pH of 2.5 and 10-2 M. This made thick layers of both PAH and PAA, but the film growth turned
nonlinear after 10 bilayers. The thinnest layer in the PAHIPAA systems, 29 A per bilayer, was
produced by the condition of PAH with a pH of 2.5 and 10-2 M and PAA with a pH of 4.5 and
10-2 M. The lower concentration of PAH and PAA solutions with 10- 3 M produced 32
A per
bilayer.
In Table 3.2, we can see that a pH of 2.5 for the PAA solution produced about 30
while a pH of 4.5 of PAA produced about 16
A regardless
A
of the PAH solution conditions. The
Table 3.2. Layer thickness of PAHIPAA with different solution conditions (*** no data
generated).
Condition of
Condition of
PAH layer
PAA layer
Total Thickness
PAH Solution
PAA Solution
contribution to
bilayer
contribution to
bilayer
per bilayer
10- 2 M, pH 2.5
10- 2 M, pH 2.5
13
2A
29±2 A
42 + 3 A
10- 2 M, pH 2.5
10- 2 M, pH 2.5
28 + 3 A
30 ± 3A
58 + 4 A
(MgCl2 0.4 M)
10- 2 M, pH 4.5
10- 2 M, pH 2.5
39 + 3 A
33 ± 2 A
72 ± 3 A
10- 2 M, pH 4.5
10- 2 M, pH 4.5
47 + 2 A
18 ± 2 A
65 ± 3 A
10-2 M, pH 2.5
10-2 M, pH 4.5
14 ± 3 A
15 + 3 A
29 ± 3 A
10- 3 M, pH 2.5
10- 3 M, pH 2.5
***
***
32 + 2 H
same trend was observed in the thickness data for a pH of 2.5 and 4.5 for the PAH solution. The
thickness of PAH layers fabricated at the pH 2.5 was about 15 A and that fabricated at the pH 4.5
was in the range of 40 A. These results suggest that the thickness of previously deposited layers in
the PAH/PAA systems may not affect the additional layer thickness of the oppositely charged
layers. This phenomena will be discussed in the discussion section in greater detail.
0.2
0.15
0.1
0.05
0
1 2
3
4 5 6 7 8 9 10 11 12
Number of Layers
Figure 3.9. The amount of methylene blue dye adsorption onto PAH/PAA layers
fabricated by pH 2.5, 10-2 M of PAH with MgCl2 0.4 M and pH 2.5, 10-2 M of PAA
solutions.
A simple dying technique was introduced to give a relative measure of the amount of
carboxylic acid groups on the surface. By dying the adsorbed layers with positively charged
methylene blue, the intensity of the methylene blue absorption indirectly indicates the amount of
negative charge on the surface. Because the dye molecule may, however, diffuse into the
multilayer films, the methylene blue dipping time was controlled to be rather short.
The
consecutive layers from 7 through 10 were dipped into the methylene blue dye solution for 10
minutes. Methylene blue did not detectably adsorb onto a bare glass slide. For the odd numbered
layers, PAH is the outermost layer, if it covers the previous layer completely, the positive charges
of the methylene blue will be repelled by the positive charges of the PAH. Figure 3.9 shows the
relative amount of the dye adsorbed onto multilayers produced with a pH of 2.5 of 10-2 M PAH
solution with 0.4 M MgCl2 and with a pH 2.5 of 10-2 M PAA solution. Like the contact angle
oscillation, the amount of methylene blue adsorption systematically varied with the outermost
layer. The amount of dye adsorbed at the different conditions are summarized in Table 3.3.
In Table 3.3, some cases show the positively charged methylene blue dye does adsorb
onto the polycationic (PAH) layers. This tends to indicate that there were some negatively charged
species on the surface which promoted dye adsorption. By comparing this result to the thickness
data, methylene blue was adsorbed onto the PAH layers, when the PAH forms rather thin layers.
Table 3.3. UV/Vis intensity of methylene blue absorbance peaks (at 600 nm).
Condition of
PAH Solution
Condition of
PAA Solution
7 layers
8 layers
9 layers
10 layers
10- 2 M, pH 2.5
10- 2 M, pH 2.5
0.021
0.147
0.0298
0.1369
10- 2 M, pH 2.5
10- 2 M, pH 2.5
< 0.001
0.12
< 0.001
0.11
(MgC12 0.4 M)
10- 2 M, pH 4.5
10- 2 M, pH 2.5
< 0.001
0.068
< 0.001
0.0644
10- 2 M, pH 4.5
10- 2 M, pH 4.5
< 0.001
0.019
< 0.001
0.018
10- 2 M, pH 2.5
10- 2 M, pH 4.5
0.0041
0.0093
0.0047
0.010
10- 3 M, pH 2.5
10- 3 M, pH 2.5
0.0169
0.081
0.025
0.0851
This indicates that thick layers of PAH limit the access of negative charges to the surface. The
largest amount of dye adsorption for an even number of layers was for the one made with a pH of
2.5 for the PAA solution, while the least amount of dye adsorption for an even number of layers
was for the one made with a pH of 4.5 for the PAH solution. We can use this technique to
estimate the level of interpenentration between layers as well as the amount of carboxylic acid
groups on the surface.
In the previous section, the number of carboxylic acid groups primarily affected the
wettability of the PAH/PAA multilayer films. By comparing the methylene blue adsorption with
the layer-by-layer contact angles, information about the interdiffusion of the layers will be
obtained. The various conditions generated different layer thicknesses of PAH/PAA multilayer
films. The surface properties of various PAH/PAA systems are possibly explained as layer
diffusions by comparing with the thickness data and the amount of methylene blue adsorption in
the layer-by-layer manner.
5.
Contact Angles of PAH / PAA multilayer films
5.1 Salt Effect
Multilayers of PAH and PAA on hydrophilic slides were fabricated with a pH of 2.5 and
10-2 M for both solutions. The contact angles for films containing from 1 to 12 layers are shown
in Figure 3.10. The films with an even number of layers produced completely wettable surfaces,
while the contact angle of the films with PAH as the outermost layer was about 25 - 27 degrees.
The thickness per bilayer of the films was in the 41 A range where an average layer increment was
29 A for PAA and 12 A for PAH. An even number of layers generated completely wettable
surfaces and the highest methylene blue adsorption. Methylene blue also adsorbed onto the
cationic outermost layer (PAH), which suggests that this thin PAH layer did not block the
adsorption of methylene blue completely.
The contact angles of PAH/PAA multilayers on hydrophilic and hydrophobic slide
substrates, with a pH of 2.5 and 10-2 M for both PAH and PAA solutions, are shown in Figure
3.3 and Figure 3.10. The same repeated oscillation of the contact angle by the even-odd pattern
was observed regardless of the wettability of the bare substrates. The different hydrophilicity of
the surfaces does not influence the wettability of the multilayer films if the first layer attaches to the
surface to render sufficient positive charges.
When 0.4 M of MgCl 2 was added to PAH solution with a pH of 2.5, the added ionic
strength increased the thickness of the bilayer to about 58
fabricated without salt by 17
A.
A, which was thicker than the sample
Figure 3.10 shows an increased oscillation of the contact angle
with added ionic strength in the PAH solution. The PAH layers with salt have higher contact
angles than the PAH layers formed without salt in the solution. In the methylene blue adsorption,
I3U
5-
50
40
30
S20
, 10
0
1
2
3
4 5 6 7 8 9 10 11 12
Number of Layers
Figure 3.10. Salt in PAH solution effect on contact angles of PAH/PAA layers
fabricated by pH 2.5, 10-2 M of solutions.
the even numbered layers attracted a lesser amount of dye, but, the odd numbered layers did not
attract the methylene blue. The thicker polycationic layer may cover the polyanionic layer more
fully and therefore reduce the amount of carboxylic acid groups on the surface with PAH as the
outermost layer.
60
50
40
30
Ion
U
0~
20
10
0
1
2
3 4 5
Number
7 8 9 10 11 12
Layers
Figure 3.11. Contact angles of PAH/PAA layers fabricated by both 10-2 M,
pH 4.5 solutions.
5.2
Solution pH Effect
We further examined effects of varying the solution pH on the PAH/PAA multilayer
films. A pH of 4.5 and 10-2 M of both PAH and PAA solutions produced the multilayer films
ranging from 1 to 12 layers. As compared to the films made with a pH of 2.5 for both solutions,
Figure 3.11 shows that the oscillation of the contact angle in the PAH/PAA layers moved to higher
values, while the amount of oscillation between an odd and even number of layers did not maintain
constant for each bilayer. The thickness of the PAH layer increased from 12 to 47 A by changing
the pH of PAH solution from 2.5 to 4.5. The PAA layers did not produce completely wettable
surfaces. Compared with the PAA layer fabricated in a pH 2.5 solution, the layer thickness
decreased from about 30 to 15 A and the methylene blue adsorption also decreased drastically. A
different pH of the solutions changes the adsorption behavior of the PAH and PAA layers and
therefore the thickness of the PAH and PAA layers were modified. The combinations of the
different pH solutions were used to produce the different thicknesses of PAH and PAA. A pH of
2.5 for the PAH solution and a pH of 4.5 for the PAA solution formed thin layers, while a pH 4.5
for the PAH solution and a pH of 2.5 for the PAA solution formed thicker layers.
Figure 3.12 shows the contact angle of PAH/PAA layers fabricated with a pH of 2.5 for
An
5
4
PC
01
tNO
3
=l
2
0
1
2
3
4
5
6
7
8
9 10 11 12
Number of Layers
Figure 3.12. Contact angles of PAH / PAA layers fabricated by pH 2.5, 10-2 M of
PAH and pH 4.5, 10- 2 M of PAA solutions.
the PAH solution and a pH of 4.5 for the PAA solution. The oscillation in contact angles was not
obtained at this condition and the thickness per bilayer was about 29
A.
The disappearance of the
contact angle oscillation suggests that each layer was not fully separate enough to generate different
wettable surface at the level of the resolution of contact angle measurements. The resolution of
contact angles was known within 10 A. In Table 3.3, the methylene blue deposited onto the PAH
and PAA layers, while PAA layers incurred more dye adsorption.
r-
5
4
3
2
01
2
3
4
5
6
7
8
9 10 11 12
Number of Layers
Figure 3.13. Contact angles of PAH / PAA fabricated by pH 4.5, 10-2 M of PAH
and pH 2.5, 10-2 M of PAA solutions.
A pH of 4.5 for the PAH and a pH of 2.5 for the PAA solution were used to form thick
PAH/PAA multilayer films. Figure 3.13 shows that a strong oscillation of the contact angles were
regained, but the PAA layers did not produce completely wettable surfaces under these conditions.
The thickness per bilayer of this film was about 72
A.
It is believed that the thicker layer of PAH
penetrated more into the PAA layer to produce less hydrophilic surfaces. The methylene blue only
adsorbed onto the PAA layers and not onto the PAH layers. The relative thickness of each layer
strongly affected the surface properties of outermost layers.
Several researchers including our group, have demonstrated that the ionic strength of the
dipping solutions influences the layer thicknesses of the multilayer films. In addition, in weak
polyelectrolytes like PAA, the pH of the dipping solutions also affected the layer thickness. In the
strong polyelectrolytes, which are relatively insensitive to the pH, the importance of the pH of the
dipping solutions is less.
For the above contact angle data of the PAH/PAA layers, a concentration of 10-2 M
dipping solutions were used. However, the next section discusses concentration effects on the
PAHIPAA system.
5.3
Concentration Effect
A concentration of 10-3 M for the PAH and PAA dipping solutions, which is ten times
more dilute than the previous experiments, was used to form multilayer films as shown in Figure
3.14. This figure shows that the contact angles varied from 10-12' to 28-34" where an even
number of layers did not generate fully wettable surfaces. The average thickness per bilayer made
under the conditions of pH 2.5 and 10-3 M for PAH and PAA was about 32 A. This indicates that
the number of carboxylic acid groups on the surface decreased when a lower concentration of the
solutions was used. The low concentration of the polyions decreased the amount of surface
coverage at first layers and decreased the polyion adsorptions. The methylene blue adsorption data
shows a very similar pattern to the pH 2.5 of 10- 2 M PAA and PAH solutions, but, the adsorption
was decreased. This is due to the fact that the thin PAH layer could not block the previous PAA
layer from attracting the methylene blue dye.
,Ž ,~
50
40
30
S20
10
Q1)Oq
0
1 2
3
4 5 6 7 8 9 10 11 12
Number of Layers
Figure 3.14. Contact angles of PAH / PAA fabricated by pH 2.5, 10-3 M of PAH and
PAA solutions.
5.4 Infrared Spectra of PAH/PAA Films
Two samples of 5 bilayer PAH/PAA films were prepared on ZnSe plates with a pH of 2.5
of PAH and PAA solutions and with a pH 4.5 of those solutions. The films were rinsed with
deionized water for 1 minute before infrared spectroscopy measurement. Figure 3.15 displays the
infrared spectra of 5 bilayers for those two cases. Comparison of the infrared spectra of the two
samples (pH 2.5 and pH 4.5 solutions) show an apparent difference in the peaks at 1560 and 1700
cm - 1 . For the sample made with a solution of pH of 4.5, most of the carboxylic acid groups
existed as the carboxylate form (-COO-) (1560 cm-1), while for that made with the pH 2.5
solution, most of carboxylic acid groups existed as free acid (-COOH) (1700 cm-1). The differing
number of charge segments in the PAA chain affects the structure of the adsorbed PAA layers.
The high concentration of carboxylate groups confirms that the pKa value of the PAA
polymer is around 4. This infrared spectra shows that the pH of 4.5 of both solutions has a higher
charge density of the PAA chain. Another interesting observation is that despite reports that
carboxylate form (pH 4.5) on the surface induces a stronger interaction with water than the free
carboxylic acid form (pH 2.5), a pH of 2.5 for both solutions formed more wettable surfaces
Cu
IC
2000
1800
1600
1400
1200
Wavenumber (cm 1)
1000
Figure 3.15. Infrared spectra of PAH / PAA films on ZnSe.
through carboxylic acid groups than a pH of 4.5. This means that a high density of less wettable
carboxylic acid sites at the interface successfully produced completely wettable surfaces.
Table 3.4 shows the relative intensity of the infrared peaks at 1560 cm -1 and 1700 cm -1 of
the PAH/PAA films as a function of time. The samples were prepared with a pH of 2.5 for the
solutions and the spectrum contained a large peak at around 1700 cm- 1 and smaller peak at around
1560 cm- 1. The 1560 cm-1 peak, which corresponds to the carboxylate form, intensified with
time. A distinct difference in the as prepared and the 1 month old sample was observed. This can
be assumed that unbound carboxylic acid groups in the PAH outermost layer reconstructed to form
ionic linkages with the oppositely charged amine groups or were just titrated at interface in air. The
contact angles of PAH/PAA films were measured after 1 month and the effect of aging the samples
was studied.
Table 3.4. Relative amount of Infrared peaks at 1560 and 1700 cm-1.
Days in the air
1560 cm-1
1700 cm -1
As prepared
33 %
67 %
14 days
53 %
47 %
28 days
62 %
38 %
5.5 Aging Behavior of PAH/PAA Multilayer Films
Samples with the number of PAHIPAA layers ranging from 1 to 12 were stored in a
plastic case under air which was the same as that for the film on ZnSe. The contact angle of the
samples was measured over a period of 1 month.
Figure 3.16 shows the contact angles of the as prepared samples as well as the samples
stored for 1 month. The oscillation in the contact angle was still observed and the wettability of the
even numbered layers did not change over 1 month. However, the contact angle of the films with
an odd numbered of layers increased after storage. This increase implies either that the density of
unbound carboxylic acid at the PAH outermost layers decreased through the increase of bound
sites or the deposition of more contaminants from the air onto unstable amine groups of PAH
layers. The amine groups on the surface are known to have a stronger tendency to interact with
AA
Cu
Q<
0
bo
0
1 2
3
4 5 6 7 8 9 10 11 12
Number of Layers
Figure 3.16. Aging effects on contact angles of PAH/PAA multilayer films
fabricated by pH 2.5, 10-2 M of PAH and pH 2.5, 10-2 M of PAA.
h
rn
Q)
Q)
k
bl)
Q)
a
\
J
Q)
bC1
E
c,
U
c,
E
O
U
0
1 2
3
4 5 6 7 8 9 10 11 12
Number of Layers
Figure 3.17. Aging effects on contact angles of PAHIPAA multilayer films
fabricated by pH 2.5, 10- 2 M of PAH with MgCl2 0.4 M and pH 2.5, 10-2 M of
PAA solutions.
particles or gases in the air than the carboxylic acid groups [3.37]. Figure 3.16 shows the case of
a thicker PAH layer which already has a reduced amount of carboxylic acid groups on the surface.
The contact angle of the PAH layers increase by a small amount up to 45 - 50" after 1 month.
6. Discussion
In the layer-by-layer sequential adsorption technique, the level of interlayer
interpenentration and the layer structure in the multilayer films are still not clearly understood. In
order to study the structure, various layer structures using the PAH/PAA system were produced.
The adsorption behavior of the PAH and PAA was examined by analyzing the surface properties.
Contact angle measurements and methylene blue adsorption were performed to monitor the level of
interdiffusion and amount of negative charges on the surface. The wettability of each layer was
sensitive to the chemical structure of the polyelectrolytes and the level of interpenentration of each
layer. The methylene blue was strongly adsorbed onto the negative charges on the surface. To
compare the amount of the methylene blue adsorption with wettability, the peak intensity of the
UV/Vis spectra was used to determine the amount of methylene blue adsorption on the surface.
First, the mechanism of how the layer-by-layer deposition technique generates completely
wettable surfaces in the PAH/PAA system will be discussed. I propose a model (depicted in
Figure 3.18) of producing completely wettable surfaces, which is explained as follows. When a
polyion adsorbs onto a charged surface, some of its charges are not bound to the opposite charges
on the surfaces but remain unbound. During the deposition, the outermost layer, which may
develop a loose and swollen form due to the solubility of the unbound polyion segments, are ready
to bind to oppositely charged polyions. A drying station induces the collapse of the loose and
swollen layer to a densified surface layer structure. The adsorbed structures, which are classified
in terms of trains, loops and tails, can highly influence the layer thickness and the level of layer
interdiffusion after drying. If the outermost layer has high density of loops and tails, it will likely
cover the previous layers rather completely. This model suggests that although two layers may
interdiffuse, the surface can still be dominated by the outermost layer through the collapse of this
layer.
A pH of 2.5 for the PAH solution formed a thin and extended chain (train) structure layer,
but many unbound charged sites still remained on the PAH layer. A pH of 2.5 for the PAA
solution was adsorbed onto the already deposited PAH layer with loops and tails. The loopy and
thick PAA layer produced a completely wettable surface. On the other hand, a pH of 4.5 for the
PAH solution formed a thick PAH layer, while a pH of 4.5 for the PAA solution produced thin
and extended PAA layers. The thin and extended structure of the PAA layer might be heavily
interdiffused with the thick PAH layer which consequently produced a less wettable surface. As
discussed previously, different hydrophilicities originated from the number of carboxylic acid
A
C
(
Figure 3.18. A proposed model of the layer-by-layer deposition.
groups on the surface and therefore not from the different hydrophilicities of the carboxylate and
free carboxylic acid groups. We can guess that the thin, extended chain configuration of the PAA
layer did not fully cover the previous PAH layer and it deposited much less carboxylic acid groups
on the surface, while the thick and loopy structure of the PAA outermost layers produced a stable
and completely wettable surface (The completely wettable surface was found to be stable for more
than 4 months in the controlled environment.). The high density of carboxylic acid groups on the
surface maintained the hydrophilicity even though some of charged groups became buried in the
layers of PAH and PAA.
The different solution pH generated either thin or thick layers of PAH and PAA in the
layer-by-layer deposition process. The layer thickness dependence on pH of the dipping solution
is possibly explained based on a theory of polyelectrolyte adsorption and the experimental results
of a monolayer adsorption onto charged surfaces.
The amount of segmental repulsion in the polymer chain, the number and strength of the
interaction sites, and the screening of charges determine the behavior of polyion adsorption [3.38].
For flexible polymer chains with weak acid groups, such as PMA and PAA, the solution pH
affects not only the dissociation of the carboxylic acid groups in the polyelectrolytes in solution but
also the charge densities of adsorbed PAA or PMA layers at the interface. In Figure 3.15., the
infrared data shows that for a pH of 4.5 in the PAA solution, many of the groups are in their
carboxylate form. When the pH of the PAA solution is higher than the pKa value of 4, most of
functional groups are ionized and have a high level of repulsion between segments as well as
between the already deposited PAA chains and the PAA chains in solution. At this high level of
dissociation, the extended chain shape of the PAA layer will likely form a thin layer. When the pH
is lower than the pKa value of the PAA, the chain has a low charge density. The low level of
segmental repulsion in the polyelectrolytes and the screening of the same charges between the
deposited polyions and polyions in solution will produce a thick and loopy structure. The
incremental thickness data in Table 3.2 shows that a pH of 2.5 for the PAA solution produced a
thick layer while a pH of 4.5 for PAA solution produced a thin layer.
2.5
.5
I/I\
I
I~
\I
I
Intrasegmental Repusion Decreases
ZpH
~,~h~-ks;r
4.5
2.5
~^~z~P~
Charges on Surface Increase
Figure 3.19. Mechanism of PAH/PAA Layer adsorption.
A pH of 2.5 or pH 4.5 of the PAH, which has a pKa value of about 10, causes the amine
groups to become fully charged and to have strong repulsive forces between the segments. The
intrasegmental repulsive forces in PAH effectively do not change over the pH range from 2.5 to
4.5. However, the thickness of the PAH layer varies from about 10 A to 40 A by changing the pH
from 2.5 to 4.5. The highly charged PAH layer adsorbs due to electro-sorption between the
carboxylic acid groups of the PAA layers and the amine groups of the PAH. The charge
compensation between the amine groups and the carboxylic acid groups are the main forces which
induced the adsorption of the PAH layer. The degree of ionization of the carboxylic acid groups of
the already deposited PAA layers are influenced by the solution pH of PAH. If the pH is above the
pKa, many carboxylic acid groups are dissociated which increases the interaction sites on the
surface. This enhances the adsorption amounts of a fully charged PAH layer. Other studies have
shown that the amount of a strong polycation adsorption is likely dependent upon the opposite
charge density of the surface [3.39]. We believe that an increase in the number of interaction sites
can enhance the amount of polycation adsorbed.
In summary, the charge density of the surface determines the amount of PAH adsorbed
while the amount of segmental repulsion in the PAH effectively remains constant. On the other
hand, the amount of intrasegmental repulsion determines the amount of PAA adsorbed while the
charge density of the already deposited PAH layer remains constant. This mechanism is depicted
in Figure 3.19 for each case.
PAH with a pH of 2.5 and PAA with a pH of 2.5 generate a thin, extended chain of PAH
and a thick, loopy chain of PAA. Some of methylene blue adsorbed on the PAH outermost layer
and small oscillation of contact angles with even-odd pattern were observed in the thick layer of
PAA and thin layer of PAH. When salt was added to the PAH solution, the oscillation of the
contact angle was enhanced and methylene blue did not adsorb onto the film with PAH as the
outermost layer. The thick PAH layer blocked the influence of the previous PAA layers. A similar
behavior was observed in the recent results by the McCarthy group [3.40]. In the reference, by
adding MnC12 to both PAH and SPS solutions, they have shown an increase in the level of
oscillation of the contact angle. The loop structures of polyelectrolytes generated the less
interpenentrated layer structure on the surface.
A pH of 4.5 in both the PAH and PAA solutions formed a thick PAH layer and a thin
PAA layer. The thin layer of PAA decreased the hydrophilic property of PAA outermost layers
and the thick layer of PAH increased the hydrophobic property of PAH outermost layers. The
methylene blue adsorption on the PAA layers with a pH of 4.5 was 10 times less than that for the
PAA layer fabricated with a pH of 2.5. The methylene blue did not deposit onto the PAH surface,
due to the complete coverage of the PAA layers by the PAH outermost layer, and a higher contact
angle of the PAH layer (about 430) was obtained. The enhanced interpenentration by the thick
PAH layer decreased the density of carboxylic acid groups on the outermost PAA layer. By
controlling the pH for the PAH and PAA solutions, we can fabricate extended chain conformations
as well as coiled conformations for both the PAH and PAA layers.
The thin and extended chain conformation of both PAH and PAA made with a pH of 2.5
for the PAH and a pH of 4.5 for the PAA solutions, does not produce an oscillation of the contact
angle due to the high interdiffusion of the two layers. After deposition, the layers are not separated
well enough to be able to observe the contact angle dependence of the layers. The methylene blue
adsorbed both onto the PAA and PAH outermost layers. The loopy structure for both the PAH
and PAA, made with a pH of 4.5 for the PAH and a pH of 2.5 for the PAA retained the oscillation
in the contact angle as well as the selective adsorption of methylene blue.
The proposed mechanism based on the charged polyion adsorption successfully explains
the interpenentration of two layers and the density of the carboxylic acid groups on the surface.
The oscillation of the contact angle in the PAH/PAA system followed the net hydrophilicity of the
surface in these multilayer films.
7.
Conclusion
I have demonstrated that the layer-by-layer sequential adsorption method is a simple way
to modify surface properties. The oscillation of the contact angle in the even-odd fashion, and the
selective adsorption of methylene blue dye indicated that the surface properties were dominated by
the outermost layer. The property of outermost layer, in turn, was controlled by the layer
thickness and the structure of the multilayer film. The ionic strength and pH of the dipping
solutions controlled the structure of the layers in terms of tails, loops and trains. The layer-bylayer deposition technique successfully achieved molecular level controlled multilayer films.
REFERENCES
[3.1]
Schrader, E.; Loeb, G. I. "Modem Approach to Wettability"; Plenum Press: New
York, 1992.
[3.2]
Koberstein, J. T. Mat. Res. Soc. Bull., 1996, 21, 19.
[3.3]
Briggs, D.; Kendall, C. R. Int. J. Adhesion Adhesives 1982, 2, 13.
[3.4]
Kaplan, S. L.; Rose, P. W. PlasticsEng. 1988, 44, 77.
[3.5]
Ratner, B. D. in "Biomaterials: Interfacial Phenomena and Applications, Chap.
2"; Cooper, S. L. and Peppas, N. A. Eds.; Advances in Chemistry, 199; American
Chemical Society: Washington, 1982.
[3.6]
Briggs, D. in "Practical Surface Analysis by Auger and X-ray Photoelectron
Spectroscopy"; Briggs, D. and Seah, M. P. Eds.; John Wiley: Chichester, 1983, p 3 8 3 .
[3.7]
Pochan, J. M.; Cerenser, L. J.; Elman, J. F. Polymer 1986, 27, 1058.
[3.8]
Boegin, H. V. "Plasma Sciennce and Technology"; Cornell University Press:
Ithaca, 1982.
[3.9]
Briggs, D.; Rance, D. G.; Dendall, C. R.; Blythe, A. R. Polymer 1980, 21,
895.
[3.10] Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Interface Sci. 1984, 132, 504.
[3.11] Grisser, H. J.; Youxian, D.; Hugher, A. E.; Genegenbach, T. R.; Mau, W. H.
Langmuir 1991, 7, 2484.
[3.12] Bee, T. G.; Cross, E. M.; Dias, A. J.; Lee, K.; Shoichet, M. S.; McCarthy, T. J.
in "Contact Angle, Wettability and Adhesion"; Mittal, K. L. Ed.; VSP: Utrecht, 1993,
p561.
[3.13] Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M. J. Am. Chem. Soc.
1977, 99, 4736.
[3.14] Gibson, H. W.; Bailey, F. C. Macromolecules 1988, 13, 34.
[3.15] Sudarshan, T. S. "Surface Modification Technologies: An Engineer's Guide";
Marcel Dekker: New York, 1989.
[3.16] Maroudas, N. G. Nature 1974, 254, 695.
[3.17] Friedrich, C.; Lauprette, F.; Noel, C.; Monnerie, L. Macromolecules 1980, 13,
1625.
[3.18] Young, T. Phil. Trans. Roy. Soc. London, 1805, 95, 65.
[3.19] Good, R. J. in "Contact Angle, Wettability and Adhesion"; Mittal, K. L. Ed.;
VSP: Utrecht, 1993, p 3 .
[3.20] Zisman, W. A. in "Contact Angle, Wettability, and Adhesion."; Advance in
Chemistry Series 43; American Chemical society: Washington D.C, 1964, p57.
[3.21] Israelchvili, N.; Gee, M. L. Langmuir 1989, 5, 288.
[3.22] Van Oss, J.; Good, R. J.; Chaudhury, M. K. Langmuir 1988, 4, 884.
[3.23] Lee, L.-H. Langmuir 1996, 12, 1681.
[3.24] Fowkes, M.; Mostaf, M. A. IEC Prod. Res. Dev. 1985, 24, 369.
[3.25] Ko, Y. C.; Ratner, B. D.; Hoffman, A. S. J. Colloid Interface Sci. 1981, 82,
25.
[3.26] Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370.
[3.27] Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927.
[3.28] Wenzel, N. Ind. Eng. Chem. 1936, 28, 988.
[3.29] Good, R. J. J. Am. Chem. Soc. 1953, 74, 5041.
[3.30] Bartell, E.; Shepard, J. W. J. Phys. Chem. 1953, 57, 211.
[3.31] Drelich, J.; Miller, J. D. J. ColloidInterface Sci. 1994, 164, 252.
[3.32] Wasserman, R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
[3.33] Whitesides, G. M.; Biebuych, H. A.; Fokers, J. P.; Prome, K. L. in "Acid-Base
Interaction"; Mittal, K. L. and Anderson , H. R. Jr. Eds.; VSP: Utrecht, 1991, p2 9 1 .
[3.34] Ferguson, G. S.; Whitesides, G. M. in "Modem Approaches to Wettability:
Theory and Applictions"; Schrader, M. E. and Loeb, G. Eds.; Plenum Press: New York,
1992, p143.
[3.35] Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides,
G. M. Langmuir 1985, 1, 725.
[3.36] Xie, X.; Gengenbach, T. R.; Griesser, H. J. in "Contact Angle, Wettability and
Adhesion"; Mittal, K. L. Ed.; VSP: Utrecht, 1993, p509.
[3.37] Gaines, G. L. Jr Nature 1982, 298, 5.
[3.38] Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.;
Vincent, B. "Polymer at Interface"; Chapman & Hall: Cambridge University Press, 1993.
[3.39] Tanaka, H.; Odberg, L.: Waigner, L.; Lindstrjm, T. J. Colloid Interface Sci.
1990, 134, 219.
[3.40] Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78.
CHAPTER 4
MULTILAYERS OF FUNCTIONAL DYE MOLECULES
AND POLYELECTROLYTES
1. Introduction
Organic dye molecules have shown great potential for applications in electronics
and optics [4.1 - 4.3]. However, they are not easily manipulated into thin layer films (pm)
without extensive chemical modification of the dye molecule due to their propensity to
aggregate and crystallize. The industrial applications of dye molecules are restricted by the
processing techniques used for an organic thin film fabrication.
The layer-by-layer deposition technique for fabricating an organic thin film has
worked with a wide variety of functional materials. Using this relatively simple approach,
our group has demonstrated that layer-by-layer deposited multilayers can be used to modify
surfaces [4.4], to create electrically conducting thin films [4.5 - 4.6] and to fabricate lightemitting devices [4.7]. This approach works with a wide variety of polycations and
polyanions, making it possible to fabricate complex multilayer heterostructures with
properties that are tunable at the molecular level.
In order to extend the versatility of this new layer-by-layer processing method, the
possibility of assembling polyelectrolytes and functional organic dyes was examined.
Decher et al and others [4.8 - 4.10] have fabricated multilayers from bolaform type
materials (molecules with a rigid core connected to ionic end groups by extended alkyl
chains) and suitable polyelectrolytes. However, this approach requires the synthesis of
molecules with specially tailored molecular architectures. This chapter will describe the
100
incorporation of small dye molecules into the multilayer films which have only a few
charges in the chemical structure. In this process, the electrostatic forces established
between ionic dye molecules and oppositely charged polymers are used to build
multilayers. The ionic attractions promote strong interlayer adhesion and a uniform and
linear multilayer deposition process. With the correct combination of an ionic dye and a
polyelectrolyte, it is possible to use these dye-based bilayer systems to construct any
number of different multilayer heterostructures.
As dye molecules are linked by only a few ionic groups, the interactions between
the polyelectrolyte and dye layers are much weaker than between polyelectrolyte layers.
The desorption of adsorbed small dye molecules in the oppositely charged polyions was,
sometimes, observed due to the weak interactions. To form uniform multilayered films by
layer-by-layer deposition, appropriate pH and salt content of dipping solutions should be
used. This is necessary to prevent the dye molecules from coming off in the oppositely
charged polyelectrolyte solutions. The fundamentals of dye multilayer films and several
examples of functional multilayer films will also be discussed in this chapter.
2. Structural Aspects of Dye Multilayer Film
The sites and number of ionic groups in a functional dye determine whether or not
it will be possible to fabricate multilayer structures. Ionic groups in a chemical structure
can dissolve the dye molecules in water and create attractive forces between the layers. The
strong interactive forces in a polyelectrolyte allow the small dye molecules to be
incorporated into multilayer films. The cationic or anionic functional dye molecules,
themselves, did not produce the multilayer films. The polyelectrolytes used in the study
were shown in Figure 4.1.
101
~WCH 2 -CH
wA
CH 3
CH 2
wCH2 -C'
NH3
-
COO
Poly(allylaminehydrochloride) (PAH)
Poly(methacrylic acid) (PMA)
wCH 2 -CH~
,wCH
COO-
2 -CH 2 -NH 2
Poly(acrylic acid) (PAA)
Poly(ethyleneimine) (PEI)
~iwCH 2
CH 3.
CHM
~CH2-CH
CH 3
CH 3
.
SO3
Poly(benzyltrimethylAmoniumtrichloride) (PVBT)
Poly(sulfonated styrene) (SPS)
Figure 4.1. Chemical structures of used polyelectrolytes.
2.1
Fluorescein Complexion Multilayer Film
A single charged dye, such as methylene blue and 8-anilino-1-naphthalene was
found to adsorb onto an oppositely charged surface, but multilayer films based on the
Figure 4.2. Chemical structure of fluorescent complexion dye.
102
material were not able to be held together. To build up multilayer films through the layerby-layer deposition, the charges of the outermost layer should be converted to make an
oppositely charged layer. In principle, more than two charge groups are required to hold
two adjacent layers together. For example, the alternate dippings of 5.10 -3 M fluorescein
complexion solution and a pH 3.5 of 5.10 -3 M poly(allylamine hydrochloride) (PAH)
produced uniform multilayer films. The chemical structure of fluorescein complexion was
shown in Figure 4.2. The molecule has five carboxylic acid groups. Figure 4.3 shows the
linear correlation of UV/Vis spectra of fluorescent complexion with number of bilayers.
This result demonstrated that the simple alternate dippings of a functional dye molecule and
a polyelectrolyte can form heterostructural multilayer films.
U.lb
S0.1
0.05
A
00 350 400 450 500 550 600 650 700
Wavelength (nm)
Figure 4.3. Linear deposition of fluorescein complexion/PAH films.
103
2.2
Bordeaux R and Crystal Violet Dyes
When a polyelectrolyte deposits onto a surface, loops and tails of the
polyelectrolyte structure form unbounded charge sites for dye layer adsorption. The
deposited dye layer acts as an adsorption site to hold the oppositely charged polyelectrolyte
layer, together. To confirm that the dye layers are linked by two adjacent layers, Crystal
Violet and Bordeaux R dye multilayer films were studied. Those have identical molecules
except at the sites of the charge groups in the dye molecule (Figure 4.4). Only Bordeaux R
produced linear growth of a film with PAH (10-2 M, pH 3.5), but Crystal Violet was not
HO
IOzNa
0 3Na
SO Na
(a)
(b)
Figure 4.4. Chemical structures of (a) Crystal Violet and (b) Bordeaux R.
suitable for multilayer formation. In Bordeaux R, the charge sites are separated enough so
that they can be shared between adjacent layers to form multilayer films. In Crystal Violet,
the charge sites are not separate. Figure 4.5 shows the UV/Vis absorbance of layer films
from 1 to 10 bilayers of Bordeaux R and PAH. The small dyes like Bordeux R
successfully formed a multilayer film sharing charges with two adjacent layers. The
structural factors, such as a number of charges, charge sites in a molecule and types of
ionic charges affect the adsorption of small dye molecules.
104
_ __
0.06
0.05
*
0.04
0.03
,
0.02
0.01
400
500
600
700
800
Wavelength (nm)
Figure 4.5. Linear deposition of Bordeaux R and PAH film.
3.
Functionalized Multilayer Films
3.1 pH indicating Azo dye Film
NH3+
N=N-
-
N=N
SO3
NH3 +
S03
Figure 4.6. Chemical structure of Congo red dye.
Congo red dye (shown in Figure 4.6) contains azo groups that allow the color of
the dye to be modified when the pH of the solution is changed [4.11]. Congo red is
known as a amphoteric molecule because it has two cationic charges (amine groups) and
two negative charges (sulfonic acid groups) in the molecules.
105
Figure 4.7 shows how the visible absorption of a multilayer thin film varies with
increasing numbers of deposited congo red/polyelectrolyte layers. The linear growth
observed with this dye system is identical to the behavior of well connected dye molecules.
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
1
2 3 4 5 6 7 8
Number of bi or trilayers
9
10
Figure 4.7. Amounts of UV/Vis absorption of Congo red dye/polyelectrolytes.
This also shows that a reproducible amount of dye is deposited during each bilayer (or
trilayer) cycle. In Figure 4.7, the amphoteric nature of the Congo red dye makes it possible
to fabricate multilayer thin films from either dye/polycation bilayers or from
polycation/dye/polyanion trilayers.
The alternate dippings of a pH 4.5 of 10-2 M
poly(vinylbenzylbutyltrimethylamonium) (PVBT) and congo red dye with 0.01 weight
percent solution utilized the sulfonic acid groups in the dye, while the poly(methacrylic
acid) (PMA) with pH 4.5 and 10-2 M used the amine groups of Congo red to build up
multilayer films. The trilayer system formed interaction sites with both amine and sulfonic
106
acid groups in the Congo red dye. In fact, the amount of dye adsorbed per complete
dipping cycle (bilayer or trilayer) was seen to be the greatest when a trilayer system was
used to build the film, suggesting that the increase of interaction sites occurs when the
Congo red dye is sandwiched in between two polymers of opposite charge. In the trilayer
system, the two consecutive polyelectrolyte layers adsorbed before the Congo red dye was
deposited onto the charged surface. The bottom two layers increased charge sites for dye
adsorption and also enhanced the interlayer attractions with the dye molecules.
wm
.•1 I
U.]0
*
0.1
0.05
.<
0
400
500
700
600
Wavelength (nm)
800
Figure 4.8. pH color change of a PAH/Congo red dye multilayer film.
Since the Congo red dye is a pH indicator dye, it is possible to deposit multilayer
thin films of this material onto various substrates and vessels thereby creating glassware
with a pH sensitive coating. Figure 4.8 displays the UV/Vis spectra of a glass slide coated
with 10 (Congo red/PAH) bilayers recorded after exposure to different pH levels. The
multilayer coating was seen to change color from blue to red as the pH changes from low to
high pH. At an intermediate pH of 4.5, both absorbing states of the dye are clearly
107
evident. It should also be noted that, unlike simple dye layers directly adsorbed to glass,
the multilayer films are remarkably robust, and can be cycled from very low to very high
pH many times without any loss of dye due to desorption. These results indicate that the
adsorbed layers are strong enough to maintain the multilayer structure over a wide pH
range.
m
'CO
.%
+
_
2
(CH 3)3N
+
,%
U2
O
C02
(CH 3 )3 N
a) Anionic porphyrin dye
b) Cationic porphyrin dye
Figure 4.9. Chemical structures of porphyrin dyes.
3.2
Porphyrin dye Multilayer Films
Porphyrin dyes are well known for their interesting electrical and optical
properties and have been considered for such applications as nonlinear optics, optical data
storage, and electrochromic devices [4.12 - 4.13]. Porphyrin dyes (shown in Figure 4.9)
with charge groups were used for a multilayer film. Figure 4.10 shows that it is possible
to use the layer-by-layer deposition process to fabricate multilayer thin films with either
cationic or anionic porphyrin dyes. In both cases, it can be seen that a linear deposition
process is observed when the porphyrin dye is deposited layer-by-layer with a polymer of
108
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
350 400 450 500 550 600 650 700 750
Wavelength (nm)
(a)
0.5
0.4
0.3
0.2
0.1
n
350 400 450 500 550 600 650 700 750
Wavelength (nm)
(b)
Figure 4.10. Layer-by-layer growth of (a) a cationic porphyrin dye/PMA multilayer and
(b) an anionic porphyrin dye/PEI multilayer. Insets show the absorbance versus number of
bilayers plots for these two systems measured at 428nm.
109
the opposite charge. The anionic porphyrin has a carboxylic acid group which is a weak
acid. To keep the charges of the carboxylic acid groups of the anionic porphyrin dyes,
both the 10- 3 M anionic porphyrin and the 5.10 -3 M of poly(ethyleneimine) (PEI) solutions
were adjusted to a pH of 7. Alternate dippings of pH 4.5 of 5.10 - 3 M PMA and 10- 3 M
cationic porphyrin dyes formed the multilayer films. Figure 4.9 shows the UV/Vis spectra
of multilayers consisting of 1-10 bilayers of cationic porphyrin, PMA, anionic porphyrin,
and PEI. As it is the case, the films obtained by this approach was very uniform and had
excellent optical quality.
Because of the planar structures, the porphyrin dyes have oriented chromophore.
The ratio of absorbance of the sample for polarized radiation parallel (rp) to the axis
direction and perpendicular (r s ) to the axis direction indicated the degree of the orientation.
The quantity D = rp / rs is called dichroic ratio to the angle of a radiation of polarized light.
If there is no preferred orientation of the dye molecules, different orientations of polarized
lights do not generate different spectra. The dichroic ratio increases with the degree of
orientation of the chromophore. The normal and 450 degree radiation angles were used to
Figure 4.11. Chemical structure of tetraphenyl porphine tetrasulfonic acid,
tetrasodium salt.
110
measure the dichroic ratio of UV/Vis spectra peak of the films. The Dichroic ratio at the
normal radiation of the 10 bilayers of cationic porphyrin/PMA and anionic porphyrin/PEI
films were 0.98 and 1.00, while that at the 450 radiation angle of those films were 1.07 and
1.05. The results indicate that the preferential orientation of the dye molecules were hardly
observed in the films, therefore, the dye molecule was randomly oriented in the thin film
structure.
U.UV
A
-
0.06
0.05
0.04
0.03
0.02
0.01
nV
300
400
500
600
700
800
Wavelength (nm)
Figure 4.12. UV/vis spectra of anionic porphyrin/PAH multilayer film (420 nm peak)
and with iron in the ring (370 nm peak).
An anionic porphyrin dye with four sulfonic acid groups (structure in Figure
4.11) also formed a clear multilayer film with PAH. The films were made from solutions
of pH 4.5 of 10-2 M PAH and 10-3 M porphyrin dye. After the deposition of 10 bilayers
on the glass slide, the PAH/porphyrin dye film was dipped into the FeC13 solution to
incorporate Fe molecules into the porphyrin rings. Figure 4.12 shows that an additional
111
peak was observed at 370 nm after the iron was successfully incorporated into the ring of
the porphyrin molecules. Like this kind of behavior, the layer-by-layer deposited film
shows a novel possibility to generate sensor applications.
3.3
Infrared Absorbing Dye Multilayer Film
The near infrared region covers the wavelengths from 700 to 2000 nm. The 750-
1300 nm region is particularly important, partially because semiconductor lasers such as
gallium-aluminum-arsenide (GaA1As) lasers use those wavelengths [4.14]. The deposition
technique makes it possible to build a multilayer thin film which specifically absorbs in this
region (the chemical structure of the infrared absorbing dye (IR dye) is shown in Figure
4.13). The absorption intensity of this thin film can be controlled by changing the number
of bilayers. Figure 4.14 shows the visible spectrum of a multilayer thin film with
(6 H2)4S03 -
(H 2)4S03
HO
NaO 3S-N=N
/
SO3Na
N=N
SO3Na
SO3Na
(b)
Figure 4.13. Chemical structure of (a) Infrared absorbing dye (IR dye)
and (b) Ponceau S dye.
112
increasing numbers of deposited dye/PAH bilayers. This technique is also able to create
multilayer heterostructures comprised of several dye bilayers that absorb in different
spectral regions.
This hetero-structured films can produce a color filter which can be tuned by
simply changing the number and type of dye/polymer bilayers added to the heterostructure.
0.25
0.2
= 0.15
0.1
0.05
0
400
500
600
700
Wavelength (nm)
800
900
Figure 4.14. Linear growth of infrared absorbing dye/PAH multilayer.
To demonstrate the ability to make thin film heterostructures from functional dye
molecules, a multilayer thin film containing two different negatively charged dyes; Ponceau
S and IR dye (the chemical structures are shown in Figure 4.13) were fabricated. These
dyes were deposited layer-by-layer as adsorbed layers alternating with the cationic
polyelectrolyte, PAH. The resultant linear deposition process was easily monitored though
UV/Vis measurements of the Ponceau S absorption band (Xmax = 506 nm) and the IR dye
absorption band (Xmax = 810 nm). Twenty layer films were prepared by alternate dipping
113
of PAH and each dye molecule.
The final structure of the multilayer film was
(IPAHIPonceau SIPAHIInfrared dye 1251)5. Figure 4.15 shows the UV/Vis spectra of a 20
layer film containing 5 layers of each dye molecule. This result suggests that with the
correct combination of ionic dyes and polyelectrolytes, it is possible to manipulate
multilayers of several dye molecules by the repeated dipping of each dye molecule and a
suitable polyelectrolyte.
0.035
0.03
0.025
0.02
0.015
0.01
0.005
U
U
400
500
600
700
800
Wavelength (nm)
900
Figure 4.15. UV/vis spectra a multilayer film containing 5 layers of Ponceau S
(510 nm) and Infrared dye 125 (810 nm).
4.
Ru complex Multilayer Films
The Ruthenium (bipyridine)3+2 , [Ru(bpy)3]+ 2 complexes shown in Figure 4.16
are well known for their novel chemiluminescent properties and excellent redox stability
[4.15 - 4.16]. The solution states of [Ru(bpy)31+
2
have been heavily examined by many
researchers. However, the solid state is not easily obtained due to poor film forming
114
properties. Since the molecule does not have two separate charges to hold adjacent layers,
the multilayer film cannot be formed through layer-by-layer deposition technique. We
therefore synthesized the tris(bathophenanthroline disulfonate ruthenium), sulfonated
[Ru(phen')3]+ 2 shown in Figure 4.16 [4.17 - 4.18]. Since this new ruthenium dye
contains six sulfonate groups, it is water soluble, does not readily crystallize and can be
multilayer deposited with various polycations. This reduced the crystallinity of the whole
complex in the solid state and made it soluble in polar solvents such as water, methanol and
dimethylsulfoxide. This enabled us to make good quality thin films by spin-coating and
also allowed us to build multilayer light emitting devices by the layer-by-layer deposition
technique.
A polymer [Ru(bpy)3]+ 2 (poly([Ru(pby)3]+ 2 )ester) was prepared to enhance the
Ru polypyridyl complex adsorption in the multilayer films. Refer to chapter 2 for details
describing the structure and synthesis routes. The device behavior based on sulfonated
[Ru(phen') 3]+ 2 and poly([Ru(pby)3]+ 2 ) ester will be discussed in chapter 5.
2+
SO3N] 2+
2xSo 3 Na
U"-
(a)
(b)
Figure 4.16. The chemical structures of (a) [Ru(bpy)3]+
[Ru(phen')3]+2.
115
2
and (b) sulfonated
4.1.
Sulfonated [Ru(phen')3] +2 / Polyelectrolyte Multilayer Film
The layer-by-layer deposited multilayer films were produced by alternate dipping
of polycations such as PAH and the sulfonated [Ru(phen')3] +2 solution. Solutions, 10- 3
M of [Ru(phen')3]+ 2 and 5-10 - 3 M of PAH were prepared for the [Ru(phen') 31+ 2 /PAH
multilayer films through the automatic dipping method. Table 4.1 displays the intensity of
UV/Vis spectra of 5 and 10 bilayers of sulfonated [Ru(phen')3]+ 2 /PAH when pH of those
dipping solutions were adjusted to be 2.5 and 4.5. The first two cases in Table 4.1 show
the peak intensity of the sulfonated [Ru(phen')3]+ 2 adsorption in 5 and 10 bilayers on the
glass slides without the first bilayer of PEI/SPS on the substrate. The PEI/SPS bilayer is
often used to generate the ionic charges on the surface of hydrophilic glass. The latter two
cases in Table 4.1 shows the sulfonated [Ru(phen')3]+ 2 adsorption on the substrates with
the addition of a PEI/SPS layer. The pH of the dipping solutions did not change the
adsorption of [Ru(phen')3] + 2 . In Table 4.1, it can be seen that the dye adsorption initially
increased with the presence of the PEI/SPS layers, however the adsorption of 10 bilayers
was not dependent on the bottom PEI/SPS layers.
The sulfonic acid groups of
[Ru(phen')3]+ 2 and amine groups of PAH are strong acids and bases. As a result, the pH
2 films.
Table 4.1. Intensity of UV/Vis absorbance peaks of PAH/[Ru(phen') 1+
3
5 bilayers
10 bilayers
PAH (pH 2.5)
w/o PEI/SPS
0.018
0.053
PAH (pH 4.5)
w/o PEI/SPS
0.019
0.052
PAH (pH 2.5)
with PEI/SPS
0.03
0.051
PAH (pH 4.5)
with PEI/SPS
0.027
0.056
116
of solutions does not affect charge densities of the PAH in the solutions and the outermost
layers.
Thickness per bilayer of the sulfonated [Ru(phen')3]+ 2 and PAH was about 15 A.
The PEI and PADA polycations also successfully formed multilayer films with sulfonated
[Ru(phen')31+ 2 . Most of the sulfonated [Ru(phen')3]+ 2 multilayer films, however, turned
into non-uniform films after 35 bilayers were deposited. It is thought that the interaction of
[Ru(phen')3]+ 2 and polyelectrolytes were weak due to the low number of charge sites and
positive charges of Ru metal in the chemical structure. To increase the strength between
layers and the stability of layer deposition, we introduced polymer type [Ru(bpy)3]+ 2 .
4.2
Poly([Ru(bpy)3]+ 2 ) ester / Polyelectrolyte Multilayer Film
Both sulfonated poly(styrene) (SPS) and poly(acrylic acid) (PAA) were
introduced to build up multilayer films of the poly([Ru(bpy)31+ 2 )ester. The positively
charged poly([Ru(bpy)31+ 2 )ester are oligomer structure type materials, which we assume
have about 3 - 4 repeat units in the polymer chain. A solution of pH 3.5 of 10-2 M SPS
and the pH 3.5 of 10-2 M poly([Ru(bpy)3]+2)ester were used to fabricate the multilayered
film. The sulfonic acids in SPS are fully charged at pH 3.5 and have a high charge
density. This creates strong intrasegmental forces. Under those conditions, SPS formed
thin and extended chains with a thickness per bilayer of about 25
A.
The carboxylic acid
groups in PAA are partially charged at pH 3.5 and thereby the intrasegmental repulsion
forces in the polymer chain are screened. The looped and coiled structure of a PAA layer
was expected at a pH of 3.5. The thickness per bilayer of poly([Ru(bpy)3]+ 2 )ester/PAA
was about 60
A.
Figure 4.17 shows the UV/Vis absorption of 20 bilayers of the
poly([Ru(bpy) 3 ]+2 )ester/PAA and the poly([Ru(bpy)3]+ 2 )ester/SPS multilayer films. The
amount of poly([Ru(bpy)31+ 2 )ester adsorption by the SPS layer is less than half of the dye
adsorption through a PAA layer. These thick and loopy layer structures enhanced the dye
adsorption.
117
0.5
0.4
0.3
o
0.2
0.1
n
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Figure 4.17. UVNis absorption of the poly([Ru(bpy)3]+ 2 )ester/PAA and the
poly([Ru(bpy)3]+2)ester/SPS films (20 bilayers)
4.3 Sulfonated [Ru(phen')3] +2 / Poly([Ru(bpy)3]+ 2 )ester Film
The negatively charged sulfonated [Ru(phen')3]+ 2 and the positively charged
poly([Ru(bpy)3]+ 2 ) ester can be bounded in order to hold the adjacent two layers to build
up multilayer films. The bottom layers of the multilayer films were composed with PAH
(pH 4.5, 10-2 M) and SPS (pH 4.5, 10-2 M) deposited onto the glass slide first.
By continuous dippings of 5.10 -3 M of cationic poly([Ru(bpy)3 +2 )ester and
anionic sulfonated [Ru(phen')31+
2,
the totally ruthenium complex based films were
fabricated. In Figure 4.18, again, the pH of the dipping solutions did not influence the
adsorption of the Ru polypyridyl complexes. The sulfonic acid groups and bipyridyl
groups do not change their charge densities as the solution pH is changed. Figure 4.19
shows the linear growth of multilayer films of sulfonated [Ru(phen') 3 ]+2 and
118
poly([Ru(bpy)3] +2 ) ester. The small molecular weight of poly([Ru(bpy)3] + 2 ) ester
incorporated the limited amount of sulfonated Ru complexes. Thickness per bilayer was
about 6 A.
0.08
0.06
0.04
0.02
A
00
1
2
3 4
5
6
7
Number of bilayers
8
9
10
Figure 4.18. The amount of Ru complex adsorption at pH 2.5 and pH 4.5 of
[Ru(phen')3] + 2 and poly([Ru(bpy)3]+ 2 )ester solutions.
The poly([Ru(bpy)31+ 2 ) ester and sulfonated [Ru(phen')31+
2
system were applied
as probe layers in Tunneling Electron Microscopy (TEM). The electron charge density of
the layer makes it possible to detect the poly([Ru(bpy)3]+ 2 ) ester/[Ru(phen') 3 ]+2 layers in
the vinyl polyelectrolyte layers. The cross-sectional image of the multilayer films had
strong contrast in the normal direction where poly([Ru(bpy)31+ 2 ) ester/[Ru(phen') 3] + 2
layers were contained in the multilayer film.
119
rA
OPM
300
400
500
600
700
800
Wavelength (nm)
Figure 4.19. UV/Vis absorbance of sulfonated [Ru(phen')3]+ 2 and
poly([Ru(bpy)3] + 2 ) ester films.
5. Conclusion
Several functional dye molecules were successfully incorporated into the layer-bylayer deposited multilayer films. The structural requirement was simply two separate
charges to hold two adjacent layers together. It was demonstrated that heterostructures
with various materials can be formed by controlling the order of dippings.
120
REFERENCES
[4.1]
Waring, D. R.; Hallas, G. "The Chemistry and Application of Dyes"; Plenum
Press: New York, 1990.
[4.2]
Gregory, P. "High-Technology Applications of Organic Colorants"; Plenum
Press: New York, 1991.
[4.3]
Ulman, A. "An Introduction to Untrathin Organic Thin Films from Langmuir-
Blodgett to Self-Assembly"; Academic Press: San Diego, 1991.
[4.4]
Yoo, D.; Rubner, M. F. SPE Prodeedings,Boston, MA, 1995, 2568.
[4.5]
Cheung, J. H.; Fou, A. C.; Rubner, M. F. Thin Solid Films 1994, 244, 985.
[4.6]
Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806.
[4.7]
Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl.
Phys. 1996, 79, 1.
[4.8]
Decher, G.; Hong, J. D. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430.
[4.9]
Decher, G.; Hong, J. D. Thin Solid Films 1992, 210/211, 831.
[4.10] Mao,G.; Tsao, Y.; Tirrell, M.; Ted Davis, H.; Hessel, V.; Ringsdorf, H.
Langmuir 1993, 9, 3463.
[4.11] BJinyai, 1. in "Indcator"; Bishop, E. Ed.; Pergamon Press: Oxford, 1972, p6 5 .
[4.12] Kampas, F. J.; Yamashita, K.; Fajer, J. Nature 1980, 284, 40.
[4.13] Hanack, M.; Lang, M. Adv. Mater. 1994, 6, 819.
[4.14] Gordon, P. F.; Gregory, P. "Organic Chemisty in Color"; Springer-Verlag:
Berlin, 1983.
[4.15] Roundhill, D. M. "Photochemistry and Photo-physics of Metal Complexes,
Chap. 5"; Plenum Press: New York, 1994.
[4.16] Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Topics in Current
Chemistry 1978, 75, 1.
[4.17] Yoo, D.; Lee, J-K.; Rubner, M. F. Mat. Res. Soc. Symp. Proc. 1996, 413,
395.
121
[4.18] Lee, J.-K.; Yoo, D.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett. 1996, 69,
1686.
[4.19] Radzilowski, L. Private Communication.
122
CHAPTER 5
NOVEL RUTHENIUM POLYPYRIDYL COMPLEXES
FOR LIGHT EMITTING DEVICES
1. Introduction
1.1 Organic Light Emitting Devices
Organic-based light emitting devices (LEDs) have attracted much attention in the
last several years because of their potential applications in large area, flat and flexible
displays that can emit a full range of colors [for review papers on organic LEDs, see 5.1 5.6]. The use of polymers and small organic molecules as active materials in LEDs holds
the promise of fabricating stable and relatively inexpensive large area displays. Some
organic LEDs are reported to show more than 4% internal quantum efficiency and to have
operating lifetimes up to 10,000 hours [5.7]. The brightness of these type organic LEDs
reached more than 10,000 cd/m 2 in the best cases [5.8], which is about 100 times brighter
than the cathode ray tube (CRT) computer screen. Although such displays still should be
improved in several respects for commercial applications, development of this field has
produced great improvements in a relatively short period of time. Industrialization may be
possible within a few years.
Several methods based on inorganic materials are currently applied for producing
electroluminescent devices [5.9], which are (1) III-V semiconductor p-n junctions, (2)
fluorescent ion doped semiconductor thin film devices, and (3) semiconductor powder
devices. As the p-n junction types are the most common method in semiconductor LEDs
123
and have some similarities to organic LEDs, the brief description of the mechanism of this
p-n type devices will be useful [5.10]. In III-V semiconductor LED devices, control of
doping level of the two adjacent materials, which are interconnected and acted as positive
and negative electrodes, produce well defined band gap between them. When a voltage is
applied by electrodes, the negative and positive charges meet at the junction of the two
materials and produce light the color of which is determined by the band-gap of the two
materials. Because the semiconductor LEDs are fabricated based on a single crystal form
of two materials, the size of an LED is limited by the size of the single crystal. A big wafer
of single crystal form of gallium arsenide is not produced yet.
The brittleness of
semiconducting materials also makes it impossible to form flexible LEDs.
In the organic-based LEDs, there are no limitations in fabricating the various
shapes, sizes and even flexible displays, where well-defined interfaces and crystal
structures are not needed due to the amorphous nature of organic emitters. The fabrication
methods for the organic LEDs are cheaper by decreasing the processing steps and be easier
by the amorphous nature. In terms of emitted colors, organic LEDs can easily produce
full-color emissions through the use of several emitting materials and mixing emitters with
other organic dopants. The low cost, full-color emission and possible application to large
and flexible displays make the organic LEDs very attractive to many researchers. Organic
LEDs, however, still need more environmental stability and better operation efficiency for
display applications. These LEDs are going to be commercialized in near future as soon as
more stable and efficient emitters are developed through a processing method to form
uniform and well-ordered thin films.
Numerous organic materials have been reported as having electroluminescent
properties. The organic emitters are categorized generally as small dye molecules and
conjugated polymers. A small dye molecule like an aluminum complex, aluminum tris(8hydroxyquinoline) (Alq3), has been used frequently as an emitter material especially with
the vacuum deposition technique [5.11 - 5.19] or spin-cast films with polymer binders
124
[5.20 - 5.22]. Consecutive evaporation forms multi-slab structures to enhance the
efficiency of the devices. The mixing of Alq3 with other dopants can be used to generate
full colors easily.
The polymer type emitters have attracted much attention, because the polymer
structure inhibits the crystallization of a spin-cast form and processing techniques for the
polymer LEDs is simper than for the small dye molecular LEDs. Many conjugated
polymers such as poly(para-phenylenevinylene) (PPV) or poly(para-phenylene) (PPP)
have been used in the LEDs [5.23 - 5.28]. Non-conjugated polymers containing emitters
in the chains have shown potential for the application in LEDs [5.29 - 5.30]. The emission
from the polymer type emitters is tuned by chemical modification of their conjugation
length. The spin-casting method is often used for fabricating LEDs based on the polymer
type emitters. This simple technique successfully produces films that are good enough for
the LED application.
Stability and purification of materials, however, still need
improvements for the device applications.
Organic LEDs are fabricated as various structures. The device structures are also
important for stable and efficient device behavior. Some structural considerations for
fabricating organic LEDs will be discussed in the next section.
1.2
Structure of Organic LEDs
In organic LEDs, the so-called injection-type LEDs, there are three different stages
; charge injection, charge transport and charge recombination for the formation of excitons.
The excitons will decay to a ground state and light will be emitted. In the first stage,
double charge carriers - electrons, holes - are injected from the cathode and the anode,
respectively into the organic layers of the devices. A balanced injection from both sides is a
prerequisite for high efficiency. The high work function metals like indium-tin oxide
(ITO), Au and Pt can be used as the anode for hole-injection. For LED applications, thin
transparent ITO is necessary for the emission of light. The low work function metals, such
125
as Al, Ca, and Mg are evaporated on the top of the organic thin films for a cathode,
facilitating electron injection. The electrodes will modify the characteristics of injection of
charge carriers to the thin films. Lower work function electrodes such as Ca and Mg may
facilitate electron injection to enhance the efficiency of devices by one or two orders of
magnitude, because the electron injection is harder than hole injection in organic materials
[5.31 - 5.32]. However, Ca and Mg are extremely unstable in air. Al is preferable for
outside device applications.
In the second stage, charge carriers migrate into the devices and excite the emitter
materials. The semiconductor LEDs which have a band gap of at least 3 eV, utilize a small
number of carriers of a high density of carriers at the interface of the p-n junction, while in
organic LEDs, most of carriers for recombination should be injected from the electrodes
because of the low density of carriers in the device at the ground state. The mobility of
charge carriers and the relative mobilities of holes and electrons influence the efficiency and
the brightness of organic LEDs. The organic materials, in general, differ in their abilities to
transport electrons and holes.
The simplest type of LEDs is a single-layer-device with one organic emitter layer.
The emitter layer transports both electrons and holes in the device. In the single layer
device, the electron and hole transport generally are not balanced and recombination is
localized at the interface between the emitter layer and electrode metals. This localization
may be accompanied by quenching at the interface or damage the interface by heat produced
at the recombination. Multilayer-type devices have more than two discrete layers, such as a
hole-transport layer and an electron-transport layer, sandwiched between injecting
electrodes. Since the hole transport layer blocks electrons and the electron transport layer
blocks holes, these multilayer structures increase the efficiency of recombining electrons
and holes in the emitter layer. Several researchers reported the increase of efficiency
through the use of heterostructures with which it is easier to balance hole and electron
transport [5.11, 5.12, 5.13, 5.33 - 5.35]. Aromatic amine based materials are known for
126
their high hole mobility and hereby frequently are used as a hole transport layer, which has
a low ionization potential and small electron affinity associated with a large energy gap
[5.36 - 5.37]. Alq3 is one of the most frequently used electron transport layer materials.
Oxadiazole and triazole derivatives are also known as the possible materials for accelerating
electron transport [5.38 - 5.39]. The bulky substituent on these materials can deter the
formation of species such as exciplexes and charge transfer complexes.
In the third stage, recombination of electrons and holes in the organic layer
generates singlet excitons. Those excitons are transferred to more stable states through
radiative decay or non-radiative decay. Stable excitons are required to generate high
efficiency LED. Among the several proposed materials for LED applications, some
materials show excellent redox stability at oxidation or reduction. However, some LED
materials showed the irreversible decomposition after the recombination. There are few
references on the stability of excited states, but understanding of possible decomposition of
excited states will help the design do more efficient devices.
The overall efficiency of organic LEDs is dependent on the balance of charge
injection and charge transport in the organic layers. Multilayered structure-type LEDs offer
more flexibility in choosing the right materials to control the charge carrier injections and
the rate of diffusion of charge carriers. However, there are few fabrication methods for
generating multilayer films and these fabrication methods have not been successful in
producing well-defined multilayer films. The fabrication method for a multilayer film will
be discussed in the next section.
1.3 Fabrication Methods for Organic LEDs
A spin-casting technique and a thermal evaporation technique under vacuum are
most-widely applied techniques for fabricating organic LEDs. The spin-casting technique
involves placing a small quantity of concentrated solution onto a substrate and rotating the
substrate to dry solvent [5.40]. An cast organic thin film is thereby produced on the
127
substrate. The thickness and the quality of thin films are dependent on the rate of rotation
of the substrate and the concentration of the solutions. The spin-casting technique is
currently applied for many different commercial products in industry because of its
simplicity. Polymer-type emitters such as PPV or PPP, are successfully built into singlelayer devices using this technique.
The spin-casting technique, however, has several limitations for application to
organic LEDs. The small emitter dye molecules easily crystallize in the solid state and
produce non-uniform thin films, when spin-cast onto the ITO. The spin-casting technique
has other limitations with regard to preparing multilayer structures. By consecutive spincasting of several solutions, the multilayer structures may be fabricated. However, during
the multiple spin-castings, the solvent used should not dissolve the bottom layers. Also,
the additional layer may not form a good interface with the bottom layer. Even though this
technique has some limitations, the technique is the most versatile way to fabricate organic
thin films.
The other widely-used method for organic LEDs is the vacuum deposition
technique. At high vacuum, the organic emitter is slowly evaporated onto the ITO surface.
The vacuum deposition technique can be used to form very uniform and very thin layers as
it hinders the crystallization of the emitters and the formation of defects in the layers. By
evaporating small organic materials in sequence, the multilayer structure with uniform and
clean interfaces can be fabricated. The first highly efficient heterostructure-organic LEDs
based on Alq3 emitters were fabricated by the vacuum deposition technique. Some
companies already announced plans to commercialize full color LEDs based on Alq3
through this technique. The technique, however, requires high vacuum and harsh
manufacturing conditions for generating the uniform films. This is somewhat contradictory
to the promise that the use of organic materials would make it possible to fabricate LEDs
via simple methods.
128
In the previous chapters, the layer-by-layer deposition technique was shown to be
a simple and easy way to fabricate a thin heterostructure film. Our group pioneered the use
of the layer-by-layer deposition technique for producing organic LEDs. Thickness and
architectures of layers are manipulated with ease by this technique. Various layers, such as
hole transport layers, electron injection layers, and emitter layers, can be incorporated into a
multilayer with molecular controlled system. The layer-by-layer deposition technique does
not require the use of toxic solvents for preparing a thin film, which are used for spincasting of conjugated polymers. The first LED was fabricated by repeated dipping of a
precursor form of PPV and poly(methacrylic acid) (PMA) and poly(sulfonate styrene)
(SPS) [5.40]. The behavior of these multilayered LEDs has shown a dependence on the
oppositely charged polyelectrolytes. The two slabs of PPV/SPS and PPV/PMA greatly
enhanced the efficiency of the LED. The multilayer PPV study demonstrated the possibility
of using the layer-by-layer deposition technique for building up heterostructure LEDs.
Like these PPV based emitting devices, heterostructure multilayer films can be obtained
through the technique.
2.
2.1.
Ru Polypyridyl Complexes as Light-Emitting Materials
Characteristics of Ruthenium Polypyridyl Complexes
Even though the organic emitter materials show full-color emission, including
blue, and have higher quantum yields, the lifetime of the devices still does not match that of
semiconducting emitters. Poor durability or poor thermal stability of organic LEDs
originate from the non-uniform device film quality or instability of the organic emitter
materials at the applied voltage. The electrochemical properties of the emitter materials at
the excitation transition, which may influence the efficiency and the lifetime of the LED,
have been little studied. However, many of these emitter materials exhibit irreversible
oxidation or reduction. For example, the cyclic voltammogram of trimer units of PPV in
129
solution (THF or acetonitrile) has shown irreversible oxidation and the quasi-reversible
reduction peaks. Similar irreversible redox processes have been observed in a soluble PPV
derivative, alkoxy-substituted PPV [5.41]. On the other hand, Alq3 is known to exhibit
several reversible reduction processes [5.42] which might account for the very stable
emitter behavior.
-7 1n
U0.
4
6 104
= 0.15
510 4 0
4104
0.1
3104 "M
1104
S0.05
11
0
400
600
700
500
(nm)
Wavelength
800
Figure 5.1. Absorbance and photoluminescence (PL) spectra of sulfonated
[Ru(phen')3] +2.
Tris-(2,2'-bipyridine)ruthenium(II) [Ru(bpy)31+ 2 are well known for their novel
chemiluminescent properties and excellent redox stability [5.43 - 5.45]. [Ru(bpy)31+ 2 can
be oxidized and reduced without decomposition or dissociation of ligands. The excited
state has long life time and shows very stable decay to a lower state. To our knowledge,
[Ru(bpy)3] +2 has not yet been studied as a possible light emitting material in thin film
devices, most likely because of its propensity to crystallize in the solid state. Crystallinity
severely hinders the formation of high quality thin films. Our group therefore synthesized
130
the tris(bathophenanthroline disulfonate ruthenium complex), sulfonated [Ru(phen')3]+ 2
shown in Figure 2.1 and examined its ability to act as an emitter in LEDs [5.46]. The
absorption maximum (%max,ab) of this compound is 470 nm as a cast film. Its emission
maximum (2Imax,em) is 628 nm.
Figure 5.1 displays the absorbance and photoluminescence spectra of a spin-cast
sulfonated [Ru(phen')31+ 2 film. The red-orange emission was observed from a single
layer of the sulfonated [Ru(phen')3]+ 2. The sulfonated [Ru(phen')31+
2
produced a good
uniform single layer and formed layer-by-layer deposited multilayer films through ionic
interactions.
0.14
5~
J
D1IU
0.12
4104 00
0.1
310 4
0.08
o*
0.06
2 104
0.04
1 104
0.02
0
n
400
500
600
700
Wavelength (nm)
800
Figure 5.2. Absorbance and photoluminescence (PL) spectra of
poly([Ru(bpy)3]+ 2 )ester.
The other Ru polypyridyl complex material, a polymer containing ruthenium (II)
tris(bipyridine) complex polymer, poly([Ru(bpy)3]+ 2)ester was recently prepared and used
as an emitter [5.47]. The chemical structure of the polymer was displayed in Figure 2.2.
131
The stability of electrochemically generated species, such as oxidized Ru(llI) and reduced
Ru(I) complexes, is expected to enhance the charge-transporting property as well as the
probability of creating excited states. The main idea of preparing a poly([Ru(bpy)3]+ 2)ester
is to increase the thin film quality by using high molecular weight materials and to enhance
the amount of Ru complex units in the layer-by-layer deposited multilayer films, compared
with the [Ru(phen')3]+ 2. Furthermore, the poly([Ru(bpy)3]+ 2)ester can be incorporated
with a variety of polyanions into the multilayer films and a study made of different
polyelectrolyte effects on LEDs.
The absorbance and photoluminescence spectra of poly([Ru(bpy)3]+ 2 )ester are
shown in Figure 5.2. The shape and maximum wavelength are almost identical in the
sulfonated [Ru(phen')3]+ 2 and the poly([Ru(bpy)3]+ 2)ester.
2.2.
Single Layer Devices of Sulfonated [Ru(phen')3] +2 and
Poly([Ru(bpy)3]+ 2 )ester
Single-layer devices of the sulfonated [Ru(phen')3]+ 2 dye were prepared by spincasting. The sulfonated [Ru(phen')31+ 2 solution in 2-methoxy ethanol was spun onto glass
substrates coated with an ITO conducting layer, followed by evaporation of a 2000
A
aluminum electrode in a vacuum of 5.10-6 Torr. The sulfonated [Ru(phen')3]+ 2 solution
was 5% by weight concentration (0.025g in 0.5 ml solvent).
Several drops of the
sulfonated [Ru(phen')3]+ 2 solution covered the 2 cm 2 substrate with 2mm ITO lines. The
substrate was spun to dry the sulfonated [Ru(phen')3] +2 solution put onto the ITO. The
spin rate of the substrate was 3000 rpm. Typical thickness of the films were measured at
780
A by profilometry.
The single-layer device showed uniform orange emission under
forward bias at 6 V, with a luminance of about 1500 nW (Figure 5.3). The intensity of
electroluminescence (EL) was proportional to the current and the EL and photoluminescent
(PL) spectra were almost the same, indicating the same origin for the emission in EL as in
132
- - - - Current
-
Light
35
1600
30
1400
1200
25
1000 r.
20
800
15
600
10
400
200
0
0
1
2
3
Voltage
4
5
6
7
(V)
Figure 5.3. Current-voltage and light-voltage behavior of spin-cast sulfonated
[Ru(phen')3] 2+ device.
- - - - Current
35
---
Light
2500
30
2000
25
20
1500
15
1000
10
500
0
0
1
2
3
4
Voltage
5
(V)
6
7
8
Figure 5.4. Current-voltage and light-voltage behavior of spin-cast
poly([Ru(bpy)3]+ 2)ester device.
133
i
PL. Light was not emitted in the reverse bias with ITO as cathode for an electron injection
and Al as anode for a hole injection. The turn-on voltage of the device was observed at 2.5
V. The thickness of the sulfonated [Ru(phen')3]+ 2 layer does not influence the turn-on
voltage of the devices.
Single-layer LEDs of a poly([Ru(bpy)3]+ 2)ester were prepared by spin-casting at
similar to the condition for the sulfonated [Ru(phen')3]+ 2 devices. Figure 5.4 shows
typical light-voltage and current-voltage curves for a ITO/poly([Ru(bpy)3]+ 2)ester/Al
device. These single-layer LEDs showed the same color of orange-red emission under
forward bias with a maximum luminance of about 80 cd/m 2 (2400 nW) at a current density
of 750 mA/cm 2 . From these luminance and current density values, the external quantum
efficiency of these single-layer devices is calculated to be about 0.02% (photons per
electron). Even though high-brightness devices were obtained using the Ru complex type
rr
Poly[Ru(bpy) 3+2]
/
\4i/rr
oja
tIMIM
:;
ep
E F
~+2
Ru(phen')+2
3
//It'
// .:
I'
i
I
480 520 560 600 640 680 720 760
Wavelength (nm)
Figure 5.5. EL spectra of sulfonated [Ru(phen')3]+ 2 and poly([Ru(bpy)3] +2)ester.
134
devices, the high current of the devices indicates the need for more efficient LEDs. It
means that leakage current and unbalanced charge mobility problems persist in the single
layered devices. The turn-on voltage of the devices was at 2.5 V, which is the same as that
of the single layer devices based on sulfonated [Ru(phen')31 +2 .
A comparison of the electroluminescences of the spin-cast of the sulfonated
[Ru(phen')3] 2+ and the poly([Ru(bpy)3]2+)ester devices is displayed in Figure 5.5. There
is only a slight difference observable in the two spectra. Neither devices emitted light at the
reverse bias.
The stability of these devices was checked by holding the voltage at 6 V. Figure
5.6 shows the changes of the current and the light intensity of the spin-cast [Ru(phen')3]+ 2
and poly([Ru(bpy)3]+ 2)ester devices at 6 V. The devices exhibited relative stable behavior,
however, the light intensity dropped below half of the maximum initial value in less than 30
minutes. The current of the poly([Ru(bpy)3]+ 2)ester devices dropped faster than that of the
sulfonated [Ru(phen')3]+2 devices. The sulfonated [Ru(phen')3] +2 single-layer device
showed slightly more stable performance than the poly([Ru(bpy)3]+ 2 )ester single-layer
device. One of the interesting observations in these devices was so-called charging effects.
The current and light intensity of the devices increased for the first two or three minutes
(shown in Figure 5.6) before degrading. Our group is still doing research in order to
understand the mechanisms of the charging effects of Ru polypyridyl complex based
devices, however, we believe this behavior is related with the generation of charged
species, such as oxidized forms and reduced forms of the Ru complexes. The Ru
complexes take some time to increase the charged species in the devices when the biased
voltage is applied. The devices produced the maximum light-out once the density of charge
species saturated in the devices. Those behavior is believed to be explained by the
mechanism of electrochemiluminescence (ECL). I will discuss a proposed model of Ru
135
40
800
35
30
600
25
20
400
15
10
200
5
0
15
10
Time (minutes)
(a)
5
20
25
0
1 An•
25
o
20
o
o
800
o
o0
0o
00
15
,0
• tt
600~"
Light
400
10
200
I
I
I
I
I
I
0
v
35
15
20
25
30
Time (minutes)
(b)
Figure 5.6. Stability data of (a) [Ru(phen')31+ 2 single-layer device and (b)
poly([Ru(bpy)3]+ 2)ester single-layer device at 6 V.
0
5
10
136
complexes LEDs based on the ECL. The charging behavior in the layer-by-layer deposited
devices is going to be discussed in the following sections.
I further investigated for making more efficient and stable devices of the
sulfonated [Ru(phen')3]+2 and poly([Ru(bpy)31+ 2 )ester through the layer-by-layer
deposition technique.
3.
Sulfonated [Ru(phen')3] +2 and Polyelectrolyte Multilayer Films
A multilayer device was fabricated by alternate dipping of pH 2.5, 10-2 M of PEI
and the [Ru(phen')3]+ 2 solution. Thirty bilayer (pair of PEI and [Ru(phen')3]+ 2 layers)
devices, about 50 nm in thickness, were prepared by the layer-by-layer deposition
technique on a glass substrate patterned with lines of ITO. This was followed by the
- - e - - Current
-
-
Light
500
50
r't't'
40
400
30
300 r
20
200
10
100
0
0
0
2
4
6
8
10
12
Voltage (V)
Figure 5.7. Current-voltage and light-voltage curves of PEI/[Ru(phen') 3 ]+2
multilayer film.
137
evaporation of an aluminum electrode onto the film. This device showed uniform orange
emission under forward bias at 10 V and a luminance of 400 nW (Figure 5.7). After
inserting insulating layers (PEI) between [Ru(phen')31+ 2 layers, the turn-on voltage of the
device increased compared to that of the single-layer device. We expected much improved
film quality by using the layer-by-layer technique, but the actual film quality was not
improved due to the weak interaction between the polyelectrolyte layers and [Ru(phen')3] +2
layers. The nature of the dipping solution for a polyelectrolyte and [Ru(phen')3]+ 2
strongly affected the film fabrication. The external quantum efficiency of the multilayer
device was about 0.004% which was actually lower than that of the single-layer device.
The emission of light was not obtained in the reverse bias.
60n
1910
50
100
40
80
30
60
20
40
10
20
1n
0v
•J
5
15
10
20
25
Time (hours)
Figure 5.8. Stability of 30 bilayer PEI/[Ru(phen') 3]+2 LEDs.
The most interesting feature of this device was its relatively high stability. The
stability of the device was checked at 8 V with a luminance of 100 nW. The luminance
138
actually increased during the first 30 minutes of operation. This phenomena was observed
in all cells in the device and is probably due to some charging effects [5.48]. It took longer
time to produce maximum light-out than the spin-cast devices. The luminance then
decreased slowly over many hours (Figure 5.8). The device kept more than 30% of the
original luminance, even after 25 hours of continuous operation. The very stable current
(about 10 mA, Figure 5.8) during the entire measurement also indicated the stability of the
device.
--- Current
--
Light
30
60
25
50
20
40
15
30
10
20
10
0
0
2
4
6
8
Voltage (V)
Figure 5.9. Current-voltage and light-voltage curves of PAH /[Ru(phen')3] + 2
multilayer film.
Figure 5.9 shows the device behavior of 30 bilayers of PAH and sulfonated
[Ru(phen')31+ 2 . By alternate dipping of 10-2 M PAH at pH 2.5 and 10-2 M sulfonated
[Ru(phen')3]+ 2 at pH 2.5, a 30-bilayer, about 450 A thick film was deposited onto ITO
and the LED was prepared. The maximum light output was only 50 nW at 10 V and the
turn-on voltage of the device was the same as the PEI/[Ru(phen') 31+2 device. The amount
139
of deposited sulfonated [Ru(phen')3]+ 2 was almost the same in those two systems. The
UV/Vis spectra revealed that the peak intensity of 30 bilayers of [Ru(phen')31+ 2/PEI was
0.15 and that of [Ru(phen') 3]+2/PAH was 0.13.
The repeated dippings of [Ru(phen')31+
2
and polyelectrolyte solutions could not
produced a uniform film of thicker than 35 bilayers. I suspect that the positive charges
(+2) of Ru in the sulfonated [Ru(phen')3] +2 may inhibit the interactions with positive
polyions, PAH and PEI. To increase the strength between layers for building up thick
films and enhance stability of layer deposition, the polymer type of [Ru(bpy)3] +2 emitter
was introduced. I further studied the poly([Ru(bpy)31+ 2)ester multilayer films to produce
LEDs.
4. Poly([Ru(bpy)3]+ 2 )ester and Polyelectrolyte Multilayer
Films
The poly([Ru(bpy)3]+ 2 )ester is a water-soluble polycation so it can be selfassembled in a layer-by-layer manner with suitable polyanions such as poly(acrylic acid)
(PAA), poly(methacrylic acid), and sulfonated poly(styrene) (SPS). The thickness and
composition of bilayer of poly([Ru(bpy)3]+ 2)ester/anionic polyions can be controlled with
the condition of polyion solution.
4.1.
Poly([Ru(bpy)3]+ 2 )ester and Poly(acrylic acid) Devices
Layer-by-layer deposited multilayers of the poly([Ru(bpy)31+
2)ester
and PAA
were fabricated by the automatic dipping method. One bilayer of poly(allylamine
hydrochloride) (PAH) and PAA was deposited as a bottom layer onto the ITO substrate to
enhance adsorption of the first layer of a poly([Ru(bpy)3]+ 2)ester. The continuous dipping
of pH 3.5, 10-2 M of poly([Ru(bpy)3]+ 2 )ester and PAA generated a uniform and a linear
growth multilayer film up to 50 bilayers. This multilayer film is composed of the
140
continuous layers of the poly([Ru(bpy)31+ 2)ester and insulating layers (PAA). The
thickness-per-bilayer of the poly([Ru(bpy)31+ 2 )ester/PAA was measured to be
approximately 60 A. At pH 3.5 of the PAA solution, the PAA had a loopy structure and
many unbounded sites were left on the outermost layer. Fifteen, twenty five and thirty five
bilayers of the poly([Ru(bpy)3]+ 2) ester and PAA were adsorbed onto the ITO substrates.
I0 nn
0.02
80
0.01
60
o*
40
-0.01
20
-0.02
0
-20 -15
-10
-5
0
5
10
Voltage (V)
15
20
Figure 5.10. Current-voltage and light-voltage curves of poly([Ru(bpy)3] +2)
ester/PAA multilayer film.
After annealing them for 10 hours at 110 "C in the vacuum oven, again, about 2000 A of Al
was deposited onto the multilayered films.
Figure 5.10 shows current-voltage and light-voltage curves of 35 bilayers of the
poly([Ru(bpy)3]+ 2)ester and PAA. Each active light emitting cell was scanned multiple
times until the light and current versus voltage curves were identical in the upward and
downward scans (for further explanation, see chapter 2). The current and light of the
141
positive voltage data (0 to 18 V) in Figure 5.10 were obtained by applying the voltage in
the forward bias multiple times first. Those of the negative voltage data (0 to - 18 V) were
generated separately by applying the voltage in the reverse bias multiple times later.
Compared with the spin-cast sample of the poly([Ru(bpy)3]+ 2)ester, the 35
bilayers of the poly([Ru(bpy)31+ 2)ester and PAA showed excellent efficiency, which was
about 100 times more efficient. Even though Al was used as a cathode, the external
quantum efficiency of the device reached approximately 2%. This behavior of the devices
approaches the best device performance reported.
The maximum brightness of the spin-cast device was 120 nW at 18 V. The
maximum light of this multilayer device did not reach that of the spin-cast devices, but were
still bright enough to see under the fluorescent lamp. It can be seen that the strong increase
of the efficiency of the device was due to the increase of the balance of the charge mobilities
between reduced and oxidized forms of the poly([Ru(bpy)31+
2) esters.
The PAA layers
may decrease charge mobilities and increase the possibility of the recombination, or may
help to produce high quality films by just decreasing the pin-holes in the device. The
insulating PAA layers increased the turn-on voltage of the device and operating voltage.
The high operating voltage facilitated the generation of irreversible excited species during
the operation. The light intensity dropped less than half of the original light intensity within
10 minutes at 16 V.
The other interesting characteristic was that light was emitted in the reversed bias
mode. In the multilayer-deposited device of the poly([Ru(bpy)3]+ 2)ester and PAA, almost
symmetric behavior was observed at forward bias and reverse bias modes. The turn-on
voltage of the device was exactly the same at the two electrodes. The efficiency of the
forward bias, however, is higher than that in reverse bias. These results indicate that the
device behavior does not depend on the work function of metals at the interfaces. The
generation of charge species was performed at both electrodes to build up p/n junction type
interfaces when the voltage applied. This kind of reversible device performance was
142
reported in the PPV film. Heeger group introduced the reversible light-emitting
electrochemical cells through mixing PPV with poly(ethyleneoxide) (PEO) and salts, while
the single layer PPV devices were strongly dependent on the metal electrodes [5.49].
MacDiarmid et al. also reported the AC operating device by coating a conducting polymer
on both sides of the emitting materials [5.50].
A.
Charging Effect
Charging effect, which is seen in the hysteresis between the upward voltage stroke
and downward voltage stroke, implied that the device was enriched with reduced and
oxidized forms at the upward voltage stroke and showed better performance at the
backward voltage stroke.
The multilayer films of PAA/poly([Ru(bpy)3]+ 2)ester showed the charging effect
when the voltage was applied. The device behavior was examined by applying the voltage
in forward bias first and in the reverse bias later without recession or in reverse bias first
and in forward bias later. In both cases, the devices exhibited a strong dependence on the
previous run. Figure 5.11 shows that current-voltage and light-voltage curves followed the
normal device behavior at forward bias and strong charging behavior was observed at the
reverse bias. Once the voltage was applied to the device, the oxidized and reduced forms
of the poly([Ru(bpy)3]+ 2)ester were generated at the interfaces of the electrodes and the
film. The oxidized or reduced forms of the species at the interface did not convert to the
neutral states immediately. When a reverse bias was applied to the device again
immediately following forward bias, the charge injection was deterred by remaining
reduced or oxidized forms of the poly([Ru(bpy)3]+ 2 )ester at the metal interfaces. The
strong charging behavior was observed in the reverse bias in Figure 5.11. Figure 5.11
shows that the device performs better when the voltage was ramped up from - 16 V to 0 V
than ramped down from 0 V to -16 V. Figure 5.12 shows the same behavior in the
143
80
70
60
50
t
40
30
20
10
0
-16 -12
-8
-4 0
4
Voltage (V)
8
12
16
Figure 5.11. Current-voltage and light-voltage curves of PAA/
poly([Ru(bpy)3] +2) ester generated by applying forward bias first.
U.U1
r.4
)
3.1
5
0.0
30
0.00
25
20
15
S-0.00
10
-0.0
5
-0 01
0
.
-16 -12 -8
-4 0
4
Voltage (V)
8
12
16
Figure 5.12. Current-voltage and light-voltage curves of PAA/
poly([Ru(bpy)31+ 2)ester generated by applying reverse bias first.
144
m
opposite direction by applying the reverse bias first and forward bias later.
The charging behavior was seen in the forward bias in Figure 5.12.
Similar to the ECL devices, the population of the oxidized and reduced forms
generate up to some level before they interact to form the excited state, but this device takes
a rather longer time to generate sufficient charge groups for light emission. The charging
effects were widely seen in the Ru polypyridyl complexes based devices. In some device
performance, it takes minutes at the same voltage to reach the maximum light output.
B.
Thickness-Dependence Behavior
120
100
80
60
40
20
0
-20
-15
-10
-5
0
Voltage
5
10
15
20
(V)
Figure 5.13. Thickness dependence on Light-voltage curves of
PAA/poly([Ru(bpy)3]+ 2)ester.
The dependence of device behavior on thickness of the multilayer
poly([Ru(bpy)31+ 2 )ester and PAA bilayers were studied. The 15, 25 and 35 bilayers of
PAAlpoly([Ru(bpy)31+ 2)ester LEDs were fabricated. Current-voltage and light-voltage
145
plots were generated by following the same method for generating plots of Figure 5.10.
Each device was scanned multiple times for only forward bias until the identical curves in
the upward and downward scans measured and later, was done for the reverse bias. The
light-voltage curves of forward bias and backward bias were shown together in Figure
5.13.
Figure 5.13 shows that the thinner-layer devices worked at lower turn-on and
operating voltage. All three devices exhibited reversible current-voltage and light-voltage
curves regardless of the thickness of the devices. The thickness-dependent device
performance indicates that the insulating PAA layers decrease the charge mobilities in the
devices and that higher voltage are needed to recombine charges to make excited states.
This spin-cast devices of the poly([Ru(bpy)31+ 2)ester did not show the thickness dependent
behavior.
600
6000
500
5000
400
4000
300
3000 i
200
2000
100
1000
0
10
.
oe
0
15
20
25
30
Number of bilayers
35
40
35
40
Figure 5.14. Efficiency and maximum light output of 15, 25, 30, 35 bilayers of
PAA/poly([Ru(bpy)31+ 2)ester devices.
146
Figure 5.14 displays the maximum light output and light-to-current ratio vs.
number of layers. The light-to-current ratio improved about 2 orders of magnitude as the
thickness of the device increased from 15 to 35 bilayers, while the maximum light output
decreased as the number of layer increased. The PAA layers in the poly([Ru(bpy)31+ 2)
ester devices successfully lowered the charge mobilities in the system and enhanced the
possibility of generating excited states.
4.2.
Poly([Ru(bpy)3]+ 2 )ester and Sulfonated poly(styrene) Devices
Fifteen, twenty five, and thirty five bilayers of poly([Ru(bpy)3]+2)ester and
poly(sulfonated styrene) (SPS) were fabricated under the same conditions as those used for
the poly([Ru(bpy)3]+ 2 )ester/PAA device fabrication. The thickness per bilayer of
poly([Ru(bpy)3]+ 2)ester/SPS was about 25
A. The poly([Ru(bpy)3]+ 2)ester/SPS system
formed thinner layers and exhibited a lower amount of poly([Ru(bpy)3]+ 2)ester adsorption
in the multilayer film.
Figure 5.15 shows the current-voltage and light-voltage curves of the 25 and 35
bilayers of poly([Ru(bpy)3]+ 2)ester and SPS devices. The 35-bilayer device also had high
turn-on and operating voltages. The thickness dependence of device behavior was also
seen in the devices. The quantum efficiency of 35 bilayers of poly([Ru(bpy)3]+ 2)ester and
SPS device was about 0.035% (514 nw/6.5 mA) at 18 V which is lower than that of the
poly([Ru(bpy)3]+ 2)ester/PAA device. However, in the poly([Ru(bpy)3]+ 2 ) ester/SPS, it
was found that in reverse bias, the devices would not emit light. One of the metal
electrodes may work as a quenching site during the operation, because current was still
observed at the reverse bias. The 15 bilayers of poly([Ru(bpy)31+ 2)ester/SPS exhibited an
ohmic behavior. The overall thickness of the 15 bilayer device was about 300 A, which
may be too thin to block some holes in this LED.
147
350
300
250
S200
OM
150
100
50
0
-15
-10
0
5
-5
Voltage (V)
10
15
Figure 5.15. Thickness dependence of poly([Ru(bpy)3]+ 2)ester/SPS
multilayer devices.
5. Poly([Ru(bpy)3]+ 2 )ester and Sulfonated [Ru(phen')3] + 2
Devices
Thirty, forty, and fifty bilayer films of poly([Ru(bpy)3]+ 2 )ester and
[Ru(phen')3] +2 were fabricated on the ITO. By continuous dippings of pH 4.5 of 5-10 -3
M of cationic poly([Ru(bpy)31+ 2) ester and anionic sulfonated [Ru(phen')31+ 2 , the totally
ruthenium complex based films were fabricated. The thickness-per-bilayer was only 6 A.
The devices are much thinner than the previous polyelectrolytes and
poly([Ru(bpy)3]+ 2)ester or sulfonated [Ru(phen')31+
2
devices. Ohmic behavior, usually
seen with poor film quality, was observed in 30 and 40 bilayer devices. Only 50 bilayers
of poly([Ru(bpy)3]+ 2)ester/[Ru(phen')3] +2 emitted some light at high voltage. Figure 5.16
shows the current-voltage and light-voltage curves of 50 bilayers of poly([Ru(bpy)3]+ 2)
148
ester/Ru(phen')3+ 2 LED. The interesting thing in the device behavior was the high turn-on
voltage. Compared with the spin-cast device of Ru polypyridyl complex based emitters,
the layer-by-layer deposited Ru-based device required nearly 10 V to start emitting light.
-e- Current
- Light
80
A#
70
35
60
30
50
25
40
20
30
15
20
10
10
5
0
0
2
4
6
8
10C 12
14
16
Voltage (V)
Figure 5.16. Current-voltage and light-volta ge curves of poly([Ru(bpy)3]+ 2)
ester/[Ru(phen')3]+ 2 LED.
6.
Discussion of Ru complexes Based LEDs
ECL cells of the Ru complexes in liquid states have been investigated for many
years. Chemical, electrochemical, photochemical stability of Ru complexes attracted much
attention as an emitter for ECL cells. Typically, in a solution state, electrogenerated
oxidized and reduced forms of the Ru complexes react to form an excited state for light
emission. With the help of coreactant in the cell, the generation of the excited states is
possible only for the annihilation reaction between the coreactant and either the oxidized or
the reduced form.
149
An ideal solid state ECL device has distinctive features over the conventional
organic light emitting devices. (1) The turn-on voltage is independent of film thickness.
(2) The turn-on voltage is equal to the band gap of the emitter. (3) The turn-on voltage is
independent of electrode metals. (4) The device is operated equally well in forward bias
and reverse bias. The main reason of those behavior is that the charge injection of the
device is facilitated by charge compensating counterions at the electrodes. Accumulation of
couterions at the electrodes lower the charge injection barriers. Once the reduced and
oxidized forms are saturated at the electrodes, the two forms react for generating the excited
states in the device. I assume that the same excited state of the poly([Ru(bpy)3]+ 2)ester and
the [Ru(phen')31+
2
in a solution state are generated in the solid state devices. The mobility
of charge species and the exact mechanism of the generation of excited states in the solid
state are still studied in our group.
There are several problems associated with Ru complex solid state ECL devices.
According to the literature, the rates of electron hopping in the Ru complexes are not the
same for the reduced and oxidized forms [5.51]. Another previous important observation
is that the conductive metal electrode (Al electrode) is prone to quench the excited states of
the Ru complexes [5.48, 5.52].
In the spin-cast films of the poly([Ru(bpy)3]+ 2 )ester and the sulfonated
[Ru(phen')31+ 2 , the turn-on voltages of the device were independent of the thickness of
layer films. The turn-on voltage was in the range of 2.5 - 3.0 V which is close to the band
gap of the Ru complexes. At the reverse bias, however, the spin-cast devices did not emit
light. This result suggests that the Ru complex, itself, has unbalanced charge transports or
does not have sufficient counterion concentration at the electrodes. I think that due to
unbalanced transports of the reduced and the oxidized forms, the metal electrode quenched
the light emission in the Ru complex materials.
The reversible light emission was observed at the multilayer of PAA/
poly([Ru(bpy)31+ 2 )ester, although the forward bias generated a slightly better performance.
150
The turn-on voltage was the same both at the forward and the reverse bias. The efficiency
of the multilayer device was much higher than that of spin-cast single layer device. It is
believed that the PAA layers in the device helped to balance the transports of the reduced
and oxidized forms by decreasing the mobility of the dominant redox form in the system.
The high efficiency may be partly due to much improved film quality with the PAA layers.
The devices exhibited strong dependence on the thickness of layers. The higher turn-on
voltage was required for the thicker layer device. Compared with the ideal solid state ECL,
it can be seen that the transportation of charge species was limited by the insulating PAA
layers in the system. The SPS/poly([Ru(bpy)3]+ 2 )ester layers exhibited the similar
thickness-dependence in the forward bias, while the light emission was quenched at the
reverse bias.
The multilayer devices operated at higher voltage than the spin-cast single layer
device. Even the turn-on voltage of the only Ru complex based poly([Ru(bpy)3]+ 2 )
ester/Ru(phen')3+ 2 multilayer device was 8 V. The ionic linkage in the multilayer may
deter the transport charge species and require high field to generate the excited states of the
Ru complexes.
The reversible light emission and the same turn-on voltage of both the forward
bias and reverse bias proved the ECL mechanism in the Ru complex devices. However,
the ECL process in the Ru complex devices was limited by the charge transport and the
counterion transport. The surrounding layers affected those transports and the quenching
at electrodes.
7.
Conclusion
I have demonstrated that the novel layer-by-layer deposition technique is a viable
way to fabricate organic LED films. The technique allows us to control heterostructures of
a multilayer and to build up different combinations of materials with molecular-level
151
control. The Ru polypyridyl complex based LEDs performed differently according to the
fabrication conditions. The layer-by-layer deposition technique was proved to form devicequality films and to achieve high efficiency devices.
152
REFERENCES
[5.1]
Holmes, A. B.; Bradley, D. D. C.; Brown, A. R.; Bum, P. L.; Burroughes, J.
H.; Friend, R. H.; Greenham, N. C.; Gymer, R. W.; Halliday, D. A.; Jackson, R. W.;
Fraft, A.; Martens, J. H. F.; Pichler, K.; Samuel, I. D. W. Synthetic Metals 1993, 55-57,
4031.
[5.2]
Conwells, E. M.; Stolka, M.; Miller, M. R. Proc. SPIE 1993, 1910.
[5.3]
Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.;
Roitman, D.; Stocking, A. Science 1996, 273, 884.
[5.4]
Bradley, D. D. C. Synth. Met. 1993, 54, 401.
[5.5]
Tang, C. W. Information Display 1996, 10, 16.
[5.6]
Friend, R. H.; Bradley, D. D. C.; Holmes, A. B. Physics Worrld 1992,
November, 42.
[5.7]
Service, R. F. Science 1996, 273, 878.
[5.8]
Murayama, R. Extended Abstracts (The 54th Autumn Meeting) of the Japan
Society of Applied Physics, 1993, 1127.
[5.9]
Bergh, A. A.; Dean, P. J. "Light Emitting Diodes"; Clarendon Press: Oxford,
1976.
[5.10] Saleh, B. E. A.; Teich, M. C. "Fundamentals of Photonics, Chap. 16."; WileyInterscience: New York, 1991.
[5.11] Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
[5.12] Tang, C. W.; VanSlyke, S. A.; Chen, C. H. Jpn. J. Appl. Phys. 1989, 65,
3610.
[5.13] Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1989, 55, 1489.
[5.14] Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 56, 799.
[5.15] Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 57, 531.
[5.16] Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl. Phys. Lett.
1994, 65, 2124.
153
[5.17] Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332.
[5.18] Burrows, P. E.; Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.; Thompson,
M. E. Appl. Phys. Lett. 1994, 64, 2718.
[5.19] Burrows, P. E., Appl. Phys. Lett. 1994, 65, 2308.
[5.20] Mori, Y.; Endo, H.; Hayashi, Y. Oyo Buturi 1992, 61, 1044.
[5.21] Kido, J.; Kohda, M.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett.
1992, 61, 2627.
[5.22] Kido, J. Appl. Phys. Lett. 1994, 64, 815.
[5.23] Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58,1982.
[5.24] Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A.
J. Nature 1992, 357, 477.
[5.25] Yu, G.; Zhang, C.; Heeger, A. J. Appl. Phys. Lett. 1988, 53, 195.
[5.26] Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Machary,
K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539.
[5.27] Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Friend, R. H.;
Gymer, R. W. Nature 1992, 356, 47.
[5.28] Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A.
B. Nature 1993, 365, 628.
[5.29] Kim, Y. H.; Kwon, S. K.; Yoo, D. ; Rubner, M. F. To be submitted.
[5.30] Zhang, C.; Von Seggern, H.; Krabel, B.; Schmidt, H.-W.; Heeger, A. J.
Synthetic Metals 1995, 72, 185.
[5.31] Parker, I. D. J. Appl. Phys. 1994, 75, 1656.
[5.32] Egusa, S.; Miura, A.; Gemma, N.; Azuma, M. Jpn. J. Appl. Phys. 1994, 33,
2741.
[5.33] Brown, A. R.; Bradley, D. D. C.; Burroughes, J. H.; Friend, R. H.; Grrenham,
N. C.; Burn, P. L.; Holmes, A. B.; Kraft, A. Appl. Phys. Lett. 1992, 61, 2793.
[5.34] Saito, S.; Aminaka, E.; Tsutsui, T.; Era, M. J. Luminescence 1994, 60-61, 902.
154
[5.35] Era, M.; Adachi, C.; Tsutsui, T.; Saito, S. Chem. Phys. Lett. 1991, 178, 488.
[5.36] Tsutsi, T.; Aminaka, E-I.; Fujita, Y.; Hamada, T.; Saito, S. 1993, 55 - 57,
4157.
[5.37] Naka, S.; Shinno, K.; Okada, H.; Onnagawa, H.; Miyahita, K. Jpn. J. App.
Phys. 1994, 33, L1772.
[5.38] Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg, L. J. Science
1995, 267, 1969.
[5.39] Kido, J.; Ohtake, C.; Hongawa, K.; Okuyama, K.; Nagai, K. Jpn. J. Appl.
Phys. 1993, 32, L918.
[5.40] Fou, A. C.; Onitsuka, O.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl.
Phys. 1996, 79, 1.
[5.41] Lee, J.-K.; Schorock, R. R.; Baigent, D. R.; Friend, R. H. Macromolecules
1995, 28, 1996.
[5.42] Dehn, H.; Gritzner, G.; Gutmann, V. Mikrochim. Acta 1967, 3, 422.
[5.43] Peterson, J. D.; Gahan, S. L.; Ramnussen, S. C.; Rondo, S. E. Coord. Chem.
Rev. 1994, 132, 15.
[5.44] Roundhill, D. M. "Photochemistry and Photophysics of Metal Complexes";
Plenum Press: New York, 1994.
[5.45] Miller, C. J.; McCord, P.; Bard, A. J. J. Am. Chem. Soc. 1991, 7, 2781.
[5.46] Lee, J.-K.; Yoo, D.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett. 1996, 69,
1686.
[5.47] Lee, J.-K.; Yoo, D.; Rubner, M. F. Submitted in Chem. Mater. 1997.
[5.48] Lyons, C.; Rubner, M. F. To be submitted.
[5.49] Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086.
[5.50] Wang, Y. Z.; Gebler, D. D.; Lin, L. B.; Blatchford, J. W.; Jessen, S. W.;
Wang, H. L.; Epstein, A. J. Appl. Phys. Lett. 1996, 68, 894.
155
[5.51] Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem.
1986, 118, 601.
[5.52] Zhang, X.; Bard, A. J. J. Phys. Chem. 1982, 104, 2641.
156
CHAPTER 6
Conclusion, Summary, and Future Work
This work has shown that sequential layer-by-layer adsorption is a powerful and
versatile technique for development of organic thin films. This technique allows us to
precisely control the overall thickness and layer structure in a multilayer film. We can
design various heterostructures using many functional materials as building blocks. The
simplicity of this technique makes it attractive and economical for industrial applications.
During my thesis work, I expanded the technique successfully to include surface
modification and fabrication of light emitting devices. I found that pH, ionic strength, and
concentration of the dipping solutions generated different bilayer compositions and
structures and thereby influenced the performance of the functional multilayer films. I
demonstrated that the technique can be used to incorporate various functional materials,
from small dye molecules to Ru complexes, into a multilayer film.
In closing, I will briefly summarize the important contribution I have made for this
technique.
1. Summary - Surface Modification
It has been shown that the layer-by-layer sequential adsorption technique can
modify various surfaces so that they have very different properties. Specific surface
characteristics can be added to the heterostructure by controlling the functional groups that
are incorporated into the outermost layer. Modified surfaces produced with the bilayer
157
scheme were quite stable. They maintained their modified surface properties even when
immersed in a liquid. For example, completely wettable surfaces were produced with thick
layers of PAA and thin layers of PAH. These surface properties were maintained for more
than one month.
2. Summary - Control of Layer Structure
The net charge densities of a polyelectrolyte chain and a charged surface are
controlled by changing the pH and ionic strength of dipping solutions.
When a
polyelectrolyte is highly charged, it adsorbed as thin, extended-chain layer structure.
However, a polyelectrolyte that is less charged and is screened by high ionic strength, will
adsorb in a thick, loopy layer structure. Previous studies using the sequential adsorption
technique only discussed the effect of ionic strength on the thickness of an adsorbed layer.
I found that the bilayer thickness and composition also can be controlled by the pH of the
dipping solutions. In this work, weak polyelectrolytes were used for the first time in
multilayer films. Thus, I was able to observe the effect of the solution pH on the structure
of the adsorbed layer. A pH of 2.5 for PAA solution formed a thick, loopy layer, while a
pH of 4.5 for PAA solution formed a thin, extended chain layer.
The solution pH determined the charge density of an adsorbed weak
polyelectrolyte. If a polyelectrolyte in solution has fixed charge density, the amount of
adsorption increases with the number of charges on a surface. A pH of 4.5 for the PAH
dipping solution enhanced the dissociation of carboxylic acid groups of an adsorbed PAA
layer and thereby formed a thick PAH layer.
After measuring incremental thickness of PAH/PAA layers, I concluded that the
thickness of an adsorbed layer did not affect the thickness of an additional layer. The
condition of the dipping solutions primarily determined the thickness of the additional
layer.
158
3.
Summary - Small Dye Multilayer Films
I demonstrated that small dye molecules containing at least two separate charges
can be incorporated into multilayer films with polyelectrolyte layers. The structure of an
adsorbed polyelectrolyte layer determined the amount of small dye adsorption. Previous
researchers assumed that multilayer film build-up was not possible due to the small size of
typical dye molecules. Instead of small dye molecules, these groups used the bolaform
molecules, which have extended chains. The current work described a unique and a simple
way to generate multilayers containing small dye molecules.
4.
Summary - Light Emitting Device Fabrication
To my knowledge, this work describes the first attempt to fabricate light emitting
devices (LEDs) based on Ru complexes. The Ru complex materials are well known for
being very stable electrochemiluminescent (ECL) emitters in a liquid state. The spin-cast
films of the sulfonated [Ru(phen')3]+ 2 and poly([Ru(bpy)3]+ 2 )ester exhibited very bright
red emission. The multilayer film of the poly([Ru(bpy)3]+ 2 )ester/PAA produced highly
efficient and reversible ECL devices. I demonstrated that the technique can be used
successfully to fabricate uniform multilayer films for the Ru complex LEDs with molecular
level control.
Future Work
1. Surface Modification
Surface properties play a important role in various applications. Desirable surface
properties can be generated using the technique by selecting proper materials for the layers.
For example, poly(ethyleneoxide), which has excellent resistance to fouling by protein
adsorption, can be applied as an outermost layer. The previous results of our group
showed that alternate dipping of poly(aniline) and poly(ethyleneoxide) could produce a
159
multilayer film. Likewise, we can simply change the materials of the outermost layer to
modify the system for biological or chemical applications.
2. Solution pH effects on PAH/PAA Layer
I studied several solution conditions for PAH/PAA multilayer film fabrication.
The highly extended chain of the PAA layers was obtained at a pH of 4.5 and the loopy
chain layers at a pH of 2.5. By adjusting the pH of PAH and PAA solutions to unexplored
values, we can see how bilayer thickness and composition vary with solution pH. The
additional data of thickness increments at the different solution pH and ionic strength will
help us understand the exact mechanism of polyelectrolyte adsorption.
3. Structure of Layer-by-Layer Deposited Multilayer Films
The contact angle data in chapter 3 showed the layer diffusion occurred in the
PAH/PAA films. Layer interdiffusion is an important factor to consider when designing
functional multilayer films. Depth profiling through secondary-ion mass spectrometry
(SIMS) or x-ray photoelectron spectroscopy (XPS) could possibly be used to determine the
level of interdiffusion. For electrical devices, charge transport in multilayer films is
affected by the level of bilayer interdiffusion. Recent results of cross-section transmission
electron microscopy (TEM) showed the distinct contrast of Ru complex layers in the
multilayer films. Thus, TEM can be used as an additional tool for the study.
4. Small Dye Molecules in Multilayer Films
By choosing highly extended chain conformations of a polyelectrolyte, we may
increase the alignment of dye molecules in the multilayer films. Once we understand how
to orient dye molecules in multilayer films, wide applications for optics may be possible
using the layer-by-layer adsorption technique.
160
By simply adding some charge groups into a functional dye molecule, it may be
possible to build up a multilayer film containing the molecule. Thus, charge transport
materials, thermochromic materials, and electrochromic materials may be incorporated into
a multilayer film.
5. PAA/Poly([Ru(bpy)3]+ 2 )ester Multilayer Light Emitting Devices
I demonstrated that pH, ionic strength, and concentration of dipping solutions
affected layer thickness of a multilayer film. I used a single pH of 3.5 for the
poly([Ru(bpy)3]+ 2 )ester and PAA solutions for device applications.
The device
performance can be maximized by controlling the PAA layer contribution to the bilayer
composition. It will be interesting to see how much the device performance is modified by
changing the bilayer composition of the poly([Ru(bpy)31+ 2 )ester/PAA system.
6. Mechanism of Ru complexes based Light Emitting Devices
An understanding of charge transport mechanisms and excited state formation in
the Ru complex emitters will help us design brighter and more efficient LEDs. I speculated
that the mechanism of Ru complex LEDs is based on electrogenerated chemiluminescence.
I assumed that the Ru complexes, themselves, have unbalanced charge transport between
oxidized and reduced forms and that light emission of Ru complex devices is quenched by
a metal electrode (Al). Additional experiments will determine the mechanism underlying
the device behaviors.
7. Heterostructure Light Emitting Devices
Heterostructure layers containing hole or electron transport layers enhanced the
performance and efficiency of organic LEDs. By inserting hole transport layers or electron
transport layers in the Ru complex multilayer films, the efficiency and brightness of the
161
device will be enhanced. These studies will also help us understand the mechanism of the
Ru complex devices.
These are some of the many possible directions for future work.
162
ACKNOWLEDGMENTS
It has been my good fortune to have worked with, learned from, and enjoyed the
companionship of many individuals during my stay at MIT. I would like to take a moment
to thank the most immediate few who make my graduate school experience a most valuable
one.
First of all, I would like to express sincere thanks to my research supervisor, Prof.
Michael Rubner for his guidance, interest, and encouragement not only in regard to the
thesis but throughout the various stage of my doctoral program. I also thank my thesis
committee, Prof. Edwin Thomas and Prof. Kirk Kolenbrander for their advice and
perceptive comments.
Although I can no longer express my thanks, I wish to acknowledge my
indebtedness to the late Prof. Wan-Sick Ha, my undergraduate advisor. He provided me
with opportunities and experiences that will be invaluable throughout the years. Special
thanks also go to Prof. Sangyoung Kim, Prof. Sungil Hong, Prof. Seokwon Ko, Prof.
Changkun Choi, and Prof. Wonho Jo for their excellent teaching and encouraging me to
study abroad.
There are so many people in the Rubner Research Group who have contributed
either directly or indirectly to this thesis. I am thankful to have worked with such as
outstanding group of individuals whom I thank for their friendship, support, and
assistance. Special thanks to Osamu Onitsuka and Jin-Kye Lee for their constant support
and scholarly discussion. I am grateful to Marysilvia Ferreira, Josephine Cheung, Ken
Zemach, Augustine Fou, Jeff Baur, Mike Durstock, Erika Abbas, Erik Handy, Aiping Wu
and Hedi Mattoussi. Thanks also go to three undergraduates-Sarah O'Connor, Douglas
Howie, and Anand Raghunathan-for experimental assistance. I would also like to thank
Sandy Schaefer-Ung who has been tremendous supportive in many ways, to me and the
entire Research Group.
In addition, there are a number of people from outside of our groups who have
helped me through my time here. Special thanks to Janelle Gunther for her kindness and
friendship. I am grateful to Byeonghyoek Son, Bashir Dabbousi and Yun-hi Kim for
scholarly discussions and fun. Thanks also go to Libby Shaw and Tim McClure for help
163
with the Central Facilities. I would also like to thank to the 3M company, the NSF, and the
MIT center for Materials Science, who provided funding for this work.
My thanks also go to many Korean friends of mine in MIT. Special thanks to
member of the MIJI for their kind contributions and discussions. Many friends, within
MIT and beyond, have made my stay here an enjoyable one. Heejin, Jaejin, Sanghoon,
Sungjin, Taeshin, Seokwon, Ohsung, Youngchang, Hayoung, Sukyoung, Sungwhan,
Junbum, Hunwook, Jangsik, Yongchae, Taehwan, Jinsang, Jungsu, and Jaedeok, your
friendship I will cherish.
My gratitude must go to my family for their support and belief in me. Special
thanks to my parents for their endless love, consistent support, encouragement, and many
sacrifices for the last 30 years. How can I ever thank you enough? I am grateful to my
sisters, Junghee, Jinhee for their love and belief in me. I would also like to thank the new
member of my family: parents-in-law, brother-in-law, Youngseok, and sister-in-law,
Jungmo for their kindness. Thanks also go to my grandmother for her constant love. My
grandfather not only encouraged me to become a polymer scientist but also inspired me as a
respectful mentor. I am thankful to my cousins who stayed in Boston with me, Seungeok,
Jungmin, Jaemin, Dongkeun, and Jiwon, for sharing my sorrow and happiness.
Finally, I must thank my wife, Hyunmo for her love and constant support.
Without her encouragement and assistance, I could not have finished this thesis. Our four
month old son, Ian, encouraged me most during the past three months of writing, although
it was hard to manage writing the thesis and taking care of Ian. I hope I could give all my
love and support to him as I received from my family. I thank again, my wife for having
Ian.
To all these people, as well as to many not mentioned by name, I am profoundly
grateful.
To myself I seem to have been only like a boy playing on the seashore, and diverting
myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst
the great ocean of truth lay all undiscovered before me.
Issac Newton, English Physicist/Mathematician (1642 - 1727)
164